Last Updated Mar 24, 2016 5:05 AM EDT
On March 22, bombings ripped apart the airport and a subway station in Brussels, killing dozens of people less than six months after the deadly attacks in Paris. The investigation into the Paris attacks has dovetailed into the investigation of what went wrong in Brussels and seemingly linked the two tragedies.
The Islamic State of Iraq and Syria (ISIS) claimed responsibility for both the Paris attacks and the attacks at Brussels Airport and one an hour later on one of the city's metro trains, near the station of Maelbeek. Most of the victims were killed in the train blast.
The questions that remain are many.
So far, officials have identified three of the apparently four suspects, and they are the Bakraoui brothers, both Belgian citizens, and Najim Laachraoui, suspected of making the bombs for the Nov. 13, 2015, attacks in Paris and yesterday's carnage in Brussels.
Khalid El Bakraoui, 27, blew himself up in a subway station about an hour after Ibrahim El Bakraoui, 29, blew himself up at the airport. At least one other man, who later fled, accompanied Ibrahim El Bakraoui and Laachraoi to the airport. That man, seen in a surveillance image, has not been publicly identified. Investigators have not been clear about how many accomplices of the bombers they have identified.
There were reports in Belgian and French media on Thursday morning -- unconfirmed by law enforcement officials -- that Khalid El Bakraoui may have had an as-yet-unidentified accomplice in the attack on the subway system. The reports said security cameras had caught Bakraoui walking with another man carrying a large bag.
Khalid rented the apartment in the Forest neighborhood of the Belgian capital, using a false identity, that was raided by police last Friday in an operation that led to the arrest of top Paris attacks suspect Salah Abdeslam.
Laachraoui had been the subject of an intense manhunt until officials learned on Wednesday he was among the dead at the airport. Traveling under the false name Soufiane Kayal, he was documented driving from Hungary into Austria in September in a car driven by Abdeslam.
Police have been conducting raids across Belgium since the attacks, but officials have been tight lipped about who or what exactly they're searching for. Investigators have said the man seen in surveillance video accompanying Ibrahim El Bakraoui and Najim Laachraoui to the airport did not detonate his bomb and instead fled the scene.
At least 31 people were killed in the attacks, and as many as 300 were injured. Several U.S. residents remain in a Brussels hospital recovering from injuries. An American brother and sister pair, Sascha and Alexander Pinczowski, are missing after the attacks. The State Department said it does not know of any Americans killed in the Brussels attack, but several Americans are reportedly still unaccounted for.
The Latest (all times Eastern Daylight):
Rushed to act?
6:52 P.M. CBS News' Holly Williams reports that the timing of Tuesday's attacks may have been forced by the pressure Belgian police were putting on the terrorists. Last Friday, they raided a Brussels apartment and arrested one of the Paris conspirators. That raid also turned up the names of two brothers who went on to attack the airport and subway in Brussels. An apartment in a quiet Brussels suburb may have been an important clue that came too late. It was rented under a false name, according to Belgian media reports, by a man who turned out to be one of the suicide bombers -- Khalid El Bakraoui. He and his brother Ibrahim -- who blew himself up at the airport -- were Belgian citizens with a history of violent crime. But they were not linked to terrorism until last week, when a police raid turned into a gun fight at the apartment.
One suspected terrorist was killed and two others escaped. Inside the apartment, investigators said they found ammunition, an ISIS flag, and most importantly the fingerprints of Salah Abdeslam -- the alleged logistics man behind the Paris attacks. That led to Abdeslam's capture three days later, but the apartment may have also linked him with the Bakraoui brothers. The ammunition raised fears that another attack could be imminent.
But less than a week later -- before they were found or stopped -- the two brothers carried out their deadly plan. On a laptop discovered by police yesterday, Ibrahim El Bakraoui wrote that he didn't know what to do, and feared he would end up in a prison cell.
Missing government personnel
6:17 P.M. The United States says some government personnel in Brussels are unaccounted for since the deadly airport and subway attacks. State Department spokesman Mark Toner says the U.S. is still seeking to locate all its officials serving at the American missions to Belgium, the European Union and NATO, and their family members. Toner says some still haven't been reached, without providing an exact number. He noted that Belgium hasn't released the identities or nationalities of those killed or in Tuesday's bombings, or injured and hospitalized.
Two suitcases at airport
5:25 P.M. A law enforcement source told CBS News senior investigative producer Pat Milton that two suitcases loaded with explosives were found at the airport. These are the two that were detonated Tuesday by authorities. The suitcases were rendered safe by Belgium authorities. They were found at the Delta and American airlines counters.
Explosives used in attack examined
4:42 P.M. The explosive found at the house where the suspects in the Brussels attacks stayed is preferred among violent extremists in Europe because it's fairly easy to make and detonate, an expert says.
Belgium's chief prosecutor has said that investigators found 15 kilograms of triacetone triperoxide, or TATP, along with nails and other bomb-making materials at the house where the suspects in the attacks stayed before going to the airport.
TATP was used by Richard Reid, who tried unsuccessfully to detonate a bomb in his shoe during a trans-Atlantic flight in 2001, and in the Nov. 13 Paris attacks. In France, attackers packed TATP into their suicide vests and wielded assault rifles, killing 130 people.
Paris bombmaker ID'd as suicide bomber
4:28 P.M. The suspected bombmaker in the Paris attacks in November was one of two suicide bombers who targeted the Brussels airport, officials said Wednesday, in a new sign that both attacks are linked to the same ISIS cell.
Belgian authorities had been looking for Laachraoui since last week, suspecting him of being an accomplice of top Paris attacks suspect Salah Abdeslam, who was arrested on Friday.
Two officials told The Associated Press that Laachraoui's DNA was verified as that of one of the suicide bombers Tuesday, after samples were taken from remains found at the blast site at Brussels airport.
Laachraoui is believed to have made the suicide vests used in the Paris attacks, a French police official told The Associated Press, adding that Laachraoui's DNA was found on all of the vests as well as in a Brussels apartment where they were made.
ISIS' Europe plan
4:19 P.M. The Associated Press is reporting that ISIS has trained 400 fighters for European bloodshed. Their mission is to hit Europe in deadly waves of attacks, deploying interlocking terror cells like the ones that struck Brussels and Paris with orders to choose the time, place and method for maximum carnage.
The network of agile and semiautonomous cells shows the reach of the extremist group in Europe even as it loses ground in Syria and Iraq. The officials, including European and Iraqi intelligence officials and a French lawmaker who follows the jihadi networks, described camps in Syria, Iraq and possibly the former Soviet bloc where attackers are trained to attack the West. Before being killed in a police raid, the ringleader of the Nov. 13 Paris attacks claimed to have entered Europe in a multinational group of 90 fighters, who scattered "more or less everywhere."
Were Americans targeted?
3:45 P.M. U.S. officials tell CBS News correspondent Jeff Pegues they are looking into whether the airport bombers were targeting Americans, at least in part because the blasts happened near where U.S. airlines operate.
As reported on "CBS This Morning," U.S. investigators are closely analyzing the images of the suicide bombers captured by airport security cameras. It is rare for there to be such clear images of suicide bombers and investigators are said to be looking at what the source describes as the "symbolism" in the images.
Link to Bahamas
3:36 P.M. Authorities in the Bahamas say they are investigating whether one of the Brussels attackers, Khalid El Bakraoui, also had Bahamian nationality.
Interpol issued a "red notice" sometime before the attacks for Bakraoui's arrest, and it states he had dual Belgian and Bahamian citizenship.
Minister of National Security Bernard Nottage raised the issue Wednesday in Parliament, saying officials have not confirmed a link to the Caribbean island nation.
American couple missing
2:50 P.M.A relative of two young Americans missing since the Brussels attacks said the couple embraced the opportunity to live and travel in Europe.
Justin and Stephanie Shults had just waved goodbye to Stephanie's mother as she walked though airport security when bomb blasts tore through the airport Tuesday. The couple moved to Brussels for work in 2014. Stephanie's cousin Larry Newsom said "they've taken full advantage of living over there and experiencing the world. They travel every month to a new place in Europe."
Stephanie Shults, originally from Lexington, Kentucky, and her husband, from Tennessee, met in graduate school at Vanderbilt University in Nashville. Newsom said the family hoped "they've just been helping people, which is very much their nature. But we can't believe they wouldn't have checked in."
Utah missionary in coma
2:45 P.M.A Utah missionary seriously wounded during the airport bombings is now in a medically induced coma. The family of 66-year-old Richard Norby of Lehi said in a statement issued by the Mormon church Wednesday that shrapnel caused severe trauma to the man's lower leg and he also suffered second-degree burns to his head and neck.
Following a lengthy surgery, he is now expected to stay in a medically induced coma for a few days.
His family said a lengthy recovery is expected.
U.S. intel running names
2:41 P.M.U.S. intelligence officials tell CBS News they are running names - supplied by Belgian investigators - of those suspected of participating in the attacks in Brussels.
Some of the names were known to European and Belgian authorities since the attacks in Paris. An official tells CBS News that those individuals would logically have been on U.S. lists as well.
Turkey claims suspect deported
2:23 P.M.An official in the Turkish president's office said Brussels attacker Ibrahim El Bakraoui was deported from Turkey last year. The official corrected a recent account from Turkish President Recep Tayyip Erdogan, who said El Bakraoui was caught in June at the Turkish-Syrian border and deported to Belgium. The official, who asked to remain anonymous, said El Bakraoui was actually deported to the Netherlands in July.
Turkey said it warned both Belgium and the Netherlands that he was a "foreign terrorist fighter." The official said Dutch authorities later allowed him to go free because Belgian authorities could not establish any ties to terrorism. The official cannot be named because he was not authorized to speak publicly on the issue.
Brussels airport closure
2:15 P.M. Brussels airport is going to remain closed at least until Saturday because of the double bomb attack there. Airport spokeswoman Florence Muls says the date was pushed further back late Wednesday because authorities want to maintain a security perimeter until late Friday to continue their investigation into the attacks. Every day the airport is closed, some 600 flights are being cancelled or diverted to other airports close by.