BOSTON (CBSNewYork/AP) -- At least three people were killed and 144 others injured Monday after two explosions went off near the finish line of the Boston Marathon, police said.
Boston Police Commissioner Edward Davis said at an afternoon news conference that the "simultaneous explosions" occurred at 2:50 p.m. -- about three hours after the winners crossed the line.
An 8-year-old boy is among the dead, CBS 2 sister station WBZ-TV in Boston confirmed. Further details about the deceased were not immediately available.
CBS 2 reported that of those injured, seven victims were listed in life-threatening condition at Beth Israel Deaconess Medical Center. Eight others were listed in critical condition at Massachusetts General Hospital and Brigham & Women's Hospital.
"It's sheer chaos," one woman told WBZ-TV. "I can only imagine it's similar to what happened on 9/11 in New York."
Officials said earlier Monday afternoon there was no suspect in custody. However, CBS News' John Miller reported later that an individual, a Saudi national, was in custody and was reportedly being cooperative and denying involvement.
Miller reported the man was seen "acting suspiciously." He was eventually apprehended by a bystander and turned over to Boston Police and was being questioned by FBI.
"A civilian who thought this individual was acting suspiciously chased him down and tackled him, and then turned him over to Boston police saying, 'I saw this guy hanging around over there acting suspiciously and then he ran,'" Miller said.
Sources told CBS News that authorities were also studying video from a closed-circuit camera, showing a possible person of interest seen walking with two backpacks.
Watch: CBS News Correspondent John Miller
The two main blasts were caught on video. A sharp, thunderous crack can be heard, followed by a towering plume of white smoke. The explosions went off right near the finish line, sending runners to the ground.
As CBS 2's Lou Young reported Monday night, the first blast was so strong, one runner who appeared to be an older man tumbled to the ground.
Some 15 seconds later, a second explosion is seen going off about a block away.
The blasts triggered panic, as runners who had been making their way to the finish line suddenly began racing for their lives from the blast scene.
Competitors and race organizers were crying as they fled the chaos of the blasts along the route. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
First responders rushed to help victims trapped in the tangled debris. Some had limbs ripped away by the blast, officials said.
Video of the scene showed injured people and what appears to be several demolished viewing stands. The explosions took place near the Fairmount Copley Plaza Hotel.
Mass Governor Deval Patrick
"There are a lot of people down," said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.
Many people were taken away in bloody wheelchairs to the medical tent, some already dehydrated from the 26.2-mile run.
A Boston police officer was also wheeled from the course with a leg injury that was bleeding.
At sunset, downtown Boston was eerily calm. The area was closed to vehicular traffic, but people walking around were stunned about what happened, Young reported.
PEOPLE ASKED NOT TO CONGREGATE IN CROWDS
Families of victims were asked to call 617-635-4500. Anyone with any information about the cause of the blasts was asked to call 1-800-494-TIPS.
Police Commissioner Davis advised local residents to stay at home and visitors to head back to their hotels while asking individuals not to congregate in large crowds.
Boston Police Commissioner Edward Davis
"People should be calm, but they should understand that this is an ongoing event," Davis said. "We are stabilizing this situation at this time."
The police commissioner said there was no threat or information that the explosions were going to occur prior to the blasts. Law enforcement sources told CBS News the explosions appeared to have been caused by bombs placed in garbage cans.
Davis had earlier said there had been a third explosion at the JFK Library, but the Boston Police Department later indicated that incident was, in fact, an unrelated fire that broke out.
Another, separate explosion took place about an hour after the initial blasts, CBS News reported. Boston Police intentionally set off the explosion because they believed they had a detected a potential bomb, CBS News reported. The area near that explosion was clear at the time of the blast.
A recovered explosive device could prove to be a trove of forensic evidence for investigators, CBS News' Bob Orr reported.
OBAMA: 'WE WILL GET TO THE BOTTOM OF THIS'
Watch: President Obama Addresses The Situation
Addressing the situation Monday evening, President Barack Obama said he has directed the full resources of the federal government to investigate what happened.
"We don't yet have all the answers," Obama said. "People shouldn't jump to conclusions before we have all the facts. We will get to the bottom of this."
Obama also saluted police, firefighters and first responders for responding "heroically."
"We salute all those who assisted in responding so quickly and professionally," he said.
The president also sternly warned that any responsible groups will feel the "full weight of justice."
"We will find out who did this and we will hold them accountable," Obama said.
Eyewitness Tony Milazzi
WITNESSES DESCRIBE WHAT THEY SAW
Cell phone service was reportedly shut down in Boston to prevent any potential remote detonations of explosions, a law enforcement official told the Associated Press -- although cell phone companies disputed the report.
One woman, Ann Haskell, found herself halfway between the two explosions.
"Your body vibrated and we just saw black smoke, and we were trying to process, and the bomb to the next side of us, the next bomb went off," she said.
Cherie Falgoust said she was waiting for her husband, who was running in the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.
"There are people who are really, really bloody," McLean said. "They were pulling them into the medical tent."
Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.
Boston Police Commissioner Edward Davis
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
NEW YORK CITY STEPS UP SECURITY
The NYPD Steps Up Security In Wake Of Boston Attacks
In New York, officials were beefing up security following the explosions.
"I ask all New Yorkers to keep the victims and their families in your thoughts and prayers," Mayor Michael Bloomberg said. "Some of the security steps we are taking may be noticeable, including deployment of Critical Response Vehicles and additional police personnel, and others will not be. We have 1,000 members of the NYPD assigned to counter-terrorism duties, and they – along with the entire NYPD and the investments we have made in counter-terrorism infrastructure – are being fully mobilized to protect our city."
Anthony Roman of Long Island-based security consulting firm Roman & Associates told WCBS 880's Marla Diamond that authorities in large cities are typically on the highest alert level for events like a marathon.
"They remove all of the garbage cans, the mailboxes, they weld the manhole covers shut. They have spotter teams on the roofs, sniper teams. The camera system, as I said, is robust and the software analytics, robust. And there are a tremendous number of officers and concentrated manpower placed at that scene," Roman said. "It's relative to budget, manpower, training and sophistication. However, one can always slip through."
What happened in Boston has shaken the entire running community. In a statement, New York Road Runners president Mary Wittenberg said they'll continue to work hand in hand with the NYPD as they plan for upcoming events.
Share your thoughts in the comments section below...
(TM and © Copyright 2013 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2013 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)
for more features.