The young people joining violent terrorist organizations like ISIS and al-Shabab -- or taking actions inspired by their twisted ideologies -- hail from all different walks of life and are not always what you'd expect.
On June 12, 2016, 29-year-old Omar Mateen walked into Pulse, a gay nightclub in Orlando, Florida, and opened fire, killing at least 49 people and wounding 53 others. It is the deadliest mass shooting in American history.
Mateen, who worked as a security guard, was said by family members to be an aspiring police officer before his apparent radicalization.
Omar Mateen: On FBI radar
Omar Mateen, who was born in New York to Afghan parents, had been on the FBI's radar twice, once in 2013 for inflammatory comments he made to co-workers about terrorist ties, and again in 2014 regarding possible connections to the first known American suicide bomber in Syria, Moner Abu-Salha. Both cases, however, were closed.
Omar Mateen: Too few clues
"The FBI literally investigates things like that thousands and thousands and thousands of times. They simply found nothing to follow up on to open up a bigger investigation," former CIA deputy director and CBS News senior security contributor Michael Morell said.
Mateen: Allegiance to ISIS
Mateen himself called 911 during the attack, pledging allegiance to the Islamic State of Iraq and Syria (ISIS) and calling the Tsarnaev brothers, responsible for the 2013 Boston Marathon bombing, his "homeboys."
"I think that's very significant," said Morell, "because ISIS tells its followers that they must pledge bayat, they must pledge allegiance to ISIS before they die." While there's no direct evidence linking Mateen to ISIS overseas, the extremist group claimed credit after the attack and praised him. FBI Director James Comey said they were "highly confident that this killer was radicalized at least in part through the Internet."
In June 2016, 18-year-old Akram Musleh was arrested trying to board a bus from Indianapolis to New York, allegedly on his way to Morocco to join ISIS. The FBI arrested the teen, who'd expressed his support for the group extensively on social media, as well as his desire to join it. Here he stands in front of an ISIS flag, having bought one himself in 2014.
San Bernardino shooters
Married couple Tashfeen Malik (left) and Syed Farook had been radicalized "for quite some time," according to the FBI, before they went on a shooting rampage in San Bernardino, California on December 2, 2015, which left 14 dead and 21 wounded. Just before the shooting, Malik pledged her allegiance to ISIS leader Abu Bakr al-Baghdadi.
Tashfeen Malik & Syed Farook
Here they are entering the U.S. at the Chicago Airport on July 27, 2014 (Farook was U.S.-born and Malik an immigrant from Pakistan), a year and a half before committing one of the worst mass shootings in U.S. history.
Both were killed in a police shootout after the attack.
Mohammed Emwazi, more commonly known as "Jihadi John," earned his nickname by being a member of a four-person terrorist cell with British accents, known as "the Beatles."
Jihadi John: West London roots
Emwazi grew up in west London, gaining infamy from a number of videos chronicling the beheadings of captive journalists and aid workers, several of whom were American.
Jihadi John: Drone target
In November 2015, Jihadi John was targeted by a drone strike in Raqqa, ISIS' self-proclaimed capital. The drone had reportedly been tracking the terrorist for two days, waiting for a clear shot.
On the morning of November 13, 2015, U.S. military spokesman Steve Warren said officials were "reasonably certain" the strike killed him. Warren then went on to say the world was better off without Emwazi, 27.
In August 2015, two recently married Mississippi State students were arrested trying to join ISIS. Officials say that Muhammad Oda Dakhlalla, 22, and Jaelyn Delshaun Young, 20, conspired to get married, so that they could travel to Turkey and ultimately Syria, under the guise of the trip being their honeymoon.
Sweet home Alabama
Hoda Muthana, a 20-year-old woman from Birmingham, Alabama, abandoned her family and travelled to Syria in November 2014 to join the radical Islamic group ISIS.
Muthana graduated from Hoover High School in 2013, going on to study business at the University of Alabama. Her classmates describe her as quiet and reserved; not at all the sort of person you would expect to join a violent terrorist organization.
Muhammad Youssef Abdulazeez, the man authorities say killed four Marines in an attack on a military recruiting center and another U.S. military site in Chattanooga, Tennessee on July 16, 2015, was a 24-year-old, Kuwait-born engineer who had not been on federal authorities' radar until the bloodshed.
Hussnain Javid, who attended Red Bank High School with Abdulazeez, said he was popular and a member of the school's wrestling team.
Dzhokhar Tsarnaev committed a terrorist attack at the 2013 Boston Marathon, when he was just 19. He and his older brother, Tamerlan, assembled and then strategically placed bombs near the finish line, killing three people and wounding nearly 200.
At the time of the bombing, Dzhokhar was a college student at UMass Dartmouth, majoring in Marine Biology. He was also an all-star wrestler, and the captain of his high school's team. Dzhokhar was later sentenced to death for his crimes.
Tamerlan Tsarnaev was 26-years-old, when he and his brother planted bombs amidst the spectators at the 2013 Boston Marathon.
An aspiring heavyweight boxer, Tamerlan dropped out of community college -- where he was studying to become an engineer -- to focus on the sport. He was the New England Golden Gloves heavyweight champion from 2009-10; telling a Boston University graduate student publication that same year that he was working to become a naturalized citizen before the next Olympics, so that he could compete on the U.S. Boxing Team.
Tamerlan died after a car chase and shoot out with police.
Police captain's son
In the Fall of 2014, Boston Police Captain Robert Ciccolo reported his 23-year-old son, Alexander, to the FBI because he "had a long history of mental illness and... had become obsessed with Islam."
When authorities arrested him on July 4, 2015, they found rifles, handguns, machetes and several partially constructed molotov cocktails in his possession. A recent convert to Islam, Alexander Ciccolo is now accused of plotting a terrorist attack on the dorms and cafeteria at a crowded state college campus.
According to an FBI witness, Ciccolo's plans included executions of non-Muslim students, broadcast live over the internet. He also allegedly wanted to use pressure cookers like the ones used in the Boston Marathon Bombings, an event at which his father was one of the first responders.
During the course of the investigation, Ciccolo was recorded calling the June 2015 terrorist attack on a beach in Tunisia, "awesome." And authorities say that, like so many other young ISIS-inspired extremists, Ciccolo used social media to praise the terror group's work.
He appeared in a U.S. District Court, July 14, 2015, and was held without bail because authorities consider him extremely dangerous. They say he acknowledged trying to build explosives with an accelerant that would stick to the skin, making the fire more difficult to put out. After his arrest, he reportedly stabbed a nurse with a pen, during a routine medical exam.
In one of many easily accessible terrorist videos now on Youtube, American Troy Kastigar is seen here, attempting to specifically target Muslims in Minnesota.
Kastigar, a Minnesota man who converted to Islam, tells would-be recruits that Somalia is "the real Disneyland" in this now infamous "Minnesota Martyrs" video.
26-year-old Usaamah Rahim and 24-year-old David Wright were both allegedly radicalized by ISIS propaganda they came across online. Usaamah Rahim then hatched a plot to carry out an ISIS-style beheading of conservative activist and outspoken Islam critic Pamela Geller, after she sponsored a controversial cartoon contest of the Muslim Prophet Mohammed in Garland, TX.
Conflict at the CVS
On June 2, 2015, however, law enforcement intercepted a call between Wright and Rahim, in which Rahim insisted that he could no longer wait to take action; opting to "go after" Massachusetts "boys in blue," rather than his planned out-of-state target. Instead of discouraging Rahim, Wright allegedly supported the plan, telling him to prepare his will and leave his possessions to a named individual.
Rahim was fatally shot outside this Roslindale CVS, after he lunged at a police officer and an FBI agent with a large military-style knife. Rahim had been under 24-hour surveillance by the Joint Terrorism Task Force at the time of the incident.
Conspiracy in Boston
Rahim's nephew and co-conspirator -- 24-year-old David Wright of Everett, Massachusetts was arrested by authorities, June 2, 2015, for intent to "obstruct a federal investigation by destroying electronic evidence on Rahim's smartphone," according to the U.S. Department of Justice.
Authorities continue to search for the other people, with whom both Rahim and Wright communicated.
Elton Simpson is a former basketball star at Yavapai College; his assistant coach telling ABC that, while Simpson mostly kept to himself, you never would've expected him to attempt a terrorist attack.
Yet, on May 3, 2015, that's exactly what he and his roommate did. Dressed in full body armor and carrying assault rifles, the two men opened fire on a controversial contest in Garland, Texas to find the best cartoon of the Muslim Prophet Mohammed.
Simpson was killed during the attack; authorities later discovering that he had tweeted out his allegiance to ISIS the night before.
Garland, TX shooter
Nadir Soofi was Simpson's roommate in a Phoenix apartment, in addition to being his accomplice in the shooting in Garland, TX, on May 4, 2015.
Unlike Simpson, Soofi was not yet on the FBI's radar. But, like Simpson, he too was killed by a traffic officer working security at the controversial cartoon contest.
Mohamed Abdullahi Hassan
Mohamed Abdullahi Hassan, who goes by the nickname "Miski," left Minneapolis in 2008 to join al-Shabaab in Somalia.
And a recent review of social media found that Miski may have played a key role in inciting the May 4th shooting in Garland, TX; communicating back and forth with Elton Simpson several times on Twitter.
On April 23, 2015, for example, Miski tweeted, "The brothers from the Charlie Hebdo attack did their part. It's time for brothers in the #US to do their part." Simpson then tweeted back, "When will they ever learn? They are planning on selecting the best picture drawn of Rasulullah (saws) in Texas."
You know what happened next.
Douglas McAuthur McCain was the first known American jihadist to be killed fighting for ISIS.
He was born in Chicago, but later lived in both Minnesota and California. In the U.S., he was an avid basketball fan and an aspiring rapper.
McCain was killed during a clash with the Free Syrian Army in 2014. They found $800 cash and a U.S. passport on his body. He was 33-years-old when he died.
Zacharia Yusuf Abdurahman
Zacharia Yusuf Abdurahman is one of six Minnesota men of Somali descent, charged in April 2015, with traveling or attempting to travel to Syria and join ISIS.
Intelligence officials are particularly concerned about the Islamic extremist groups targeting recruits in the Twin Cities of Minneapolis and St. Paul, Minnesota.
Twin Cities terror
Adnan Abdihamid Farah, 19, is another one of the six Minnesota men charged, April 2015, with attempting to travel to Syria to join the Islamic State group.
Adnan Abdihamid Farah's parents confiscated his passport last year when it arrived in the mail; his mother later telling authorities that she feared her son would "disappear." In fact, two of Ayan Farah's sons were among the Minnesota men charged with attempting to join ISIS, this April.
"I cry all day," she told CBS Minnesota. "I don't know what happened."
Twin Cities terror
Hanad Mustafe Musse, 19, was also among the six Minnesota men of Somali descent, charged with traveling or attempting to travel to Syria to join ISIS.
So far this year, 25 people have been detained, attempting to leave the U.S. and join ISIS. The fear, then, is that those who successfully make the journey will one day return and carry out an attack on their home soil.
Twin Cities terror
Guled Omar, 20, is one of six young men of Somali descent, charged with trying to travel to Syria to join ISIS in April 2015.
A terrorist with movie star looks
Jamal Bana, a handsome 20-year-old college student from -- you guessed it -- Minnesota, was killed in Somalia in 2009, the very land his family had fled years before.
Bana left the country suddenly and was missing for several months, before his parents discovered that he had been killed on the internet.
Abdirahman Sheik Mohamud
Abdirahman Sheik Mohamud of Columbus, Ohio allegedly joined an Islamic extremist group in Syria, then returned to the U.S. after a cleric there instructed him to carry out an attack back home.
In April 2015, he was arrested on charges of both providing material support and laundering money to terrorists in the Middle East, over the past 18 months.
Mohamud pleaded not guilty to the charges above; his lawyer arguing that he's just a "normal 23-year-old kid," who knows more about basketball than bombs.
Mustafa Ali Salat
Mustafa Ali Salat is currently wanted by the FBI for providing material support to the Somalia-based terrorist organization al-Shabab.
Abdikadir Ali Abdi
Abdikadir Ali Abdi is also wanted for providing material support to the Somalia-based terrorist organization al-Shabab.
Cabdulaahi Ahmed Faarax
Cabdulaahi Ahmed Faarax is currently wanted for providing material support to al-Shabab, as well.