- Suspects involved in the plot remain at large, officials warn
- Brothers Khalid and Ibrahim El Bakraoui were suicide bombers, prosecutor says
- ISIS claimed responsibility for attacks at airport and a metro train in Brussels
(CNN)Three days after coordinated terrorist attacks killed 31 people and injured 300 more in Brussels, investigators are still searching for suspects.
ISIS claimed responsibility. Details are emerging about the men behind the attacks, their victims and the possibility that more plots could be in the works.
Here's what we know so far:
Airport: At 7:58 a.m. local time Tuesday, two suicide bombers struck the departure lounge of Brussels Airport in Zaventem about 37 seconds apart, killing at least 10 people.
One blast took place outside the security checkpoints for ticketed passengers and near the airline check-in counters, according to an airline official briefed on the situation.
Belgian federal prosecutor Frederic Van Leeuw said Wednesday that a third bomb, which he described as the "heaviest," had been left at the airport by a suspect who left the scene.
He said it exploded "a few moments after the bomb squad arrived" because the explosives were unstable, but no one had been injured "thanks to the professionalism of the authorities at the scene."
Metro station: About an hour after the airport explosions, Khalid El Bakraoui, a Brussels-born 27-year-old, detonated a suicide bomb on the Brussels subway at the end of rush hour.
The bomber, whom Van Leeuw said was identified by his fingerprints, was in the second car of a train at the Maelbeek metro station.
Authorities also believe there was a second suspect in that bombing, a senior Belgian security source told CNN. But that person's whereabouts -- or whether he was killed in the blast -- is unknown.
The station is in the heart of the city, where European Union institutions are based, making it a symbolic target for terrorists. NATO is also headquartered in Brussels.
Belgian authorities released an image of three suspects believed to have carried out the airport attack.
The man on the left is ISIS bomb-maker Najim Laachraoui, Belgium's federal prosecutor said Friday. A senior Belgian counterterrorism official told CNN that investigators believe Laachraoui was an ISIS bomb-maker.
Belgian investigators believe Laachraoui, recently identified as a suspect in November's Paris attacks, was killed at the Brussels Airport, multiple European officials told CNN.
The man in the middle is Ibrahim El Bakraoui, 29, Khalid's brother.
Authorities haven't identified the man on the right, but they say they're looking for him.
That man, Belgian authorities said, planted a bomb at the airport and left.
The man is believed to have been a guide, charged with ensuring the others carried out the attacks, according to Van Leeuw and experts.
A taxi driver who took the three suspects to the airport told investigators he picked them up at an address on Max Roos Street in the Brussels district of Schaerbeek, prompting a raid there that uncovered 15 kilograms of the explosive TATP, chemicals, a suitcase with nails and screws, an ISIS flag and other equipment meant to make explosives, Van Leeuw said.
Police searched two other addresses on the street and found nothing, but then found a computer containing the will of Ibrahim El Bakraoui in a trash can on the street, Van Leeuw said. The statement said he was "in a rush," "not knowing what to do," "being actively sought everywhere" and "not feeling safe," the prosecutor said.
He said police had arrested two people -- one in Schaerbeek, who was still being interrogated, and another who had been released after questioning.
Police raids have continued in the Brussels area, and one on Wednesday in Anderlecht resulted in the arrest of an individual whose identity has not yet been released.
The first victim to be identified was Adelma Marina Tapia Ruiz, 37, who was killed in the attack at the airport, according to Peruvian state news agency Andina.
Originally from Peru, Tapia Ruiz had lived in Belgium for six years and was at the airport with her husband and twin 4-year-old daughters waiting to board a plane when the bombs went off.
Belgian law student Leopold Hecht was killed at the metro station, his school, Universite Saint-Louis Bruxelles, said in a statement.
And Olivier Delespesse was killed in the metro explosion, said his employer, La Federation Wallonie-Bruxelles, a government ministry serving Francophone Brussels and Wallonia.
A U.S. State Department spokeswoman confirmed Friday that two Americans were among the dead.
"We offer our sincere condolences to their family and friends. Out of respect for the families involved, we have no further comment at this time," Elizabeth Trudeau said in a statement. "We continue to coordinate with the Belgian authorities in order to account for U.S. citizens in the city. Our embassy staff is providing all possible emergency consular assistance to U.S. citizens. Privacy considerations prevent us from speaking about any specific case."
"The Belgians have been sitting on a ticking time bomb," one U.S. counterterrorism official said.
U.S. intelligence officials say they weren't surprised that Brussels was attacked, because of concerns about terror threats, particularly after recent raids and the arrest of Paris attacks suspect Salah Abdeslam last week in Brussels.
Belgium has been a focus for counterterrorism officials for years because of the large number of Belgian foreign fighters who have traveled to join ISIS and other terror groups in Syria and Iraq -- more per capita than any other European Union country.
Many have been returning.
On March 18, after more than four months on the run, Abdeslam was captured after being wounded in a gunfight with police in the Brussels area of Molenbeek. Days later, Belgium and French authorities warned of more attacks.
Belgian officials raised the country's terror threat warning to its highest level: four. They lowered the alert level to three Thursday, meaning the threat is not imminent but remains serious, officials said. Belgium, according to the government's crisis center, is now facing "a serious, possible and likely threat."