BRUSSELS — (CNN) One terrorist blew up himself up at an airport. His brother did the same thing at a busy subway station.
And perhaps most puzzling, a man suspected of being a skilled ISIS bomb maker sacrificed his life while a fourth terrorist simply planted a bomb and left.
That’s what authorities believe happened Tuesday morning when the group of attackers devastated Brussels, killing 31 people and wounding 270 others.
Three of the terrorists appear on surveillance footage at Brussels’ international airport — nonchalantly pushing luggage carts shortly before the carnage ensued.
Now, as more clues emerge about the deceased suspects’ terrorist ties, authorities are scrambling to find a fourth — and possibly a fifth — suspect before another potential attack.
Here’s what we know about the Brussels terror suspects:
Najim Laachraoui: The suspected bomb-maker
Even at age 24, Najim Laachraoui seemed disturbingly well versed in terror plots and bomb making.
Laachraoui was born in Morocco, but brought up in Belgium. According to Belgian media, he studied electromechanical engineering at a college in Brussels.
One day before he died Tuesday, authorities named Laachraoui as a suspect wanted in connection with the Paris terror attacks that killed 130 people in November.
Investigators believe Laachraoui was a key figure in the Paris attacks, not only as the likely bomb maker, but also as a coordinator of the violence.
Laachraoui was seen on surveillance video at a Brussels Western Union office sending money to the cousin of Abdelhamid Abaaoud — the suspected ringleader of the Paris attacks — in the days leading up to those attacks.
Both Abaaoud and his cousin were later killed in a raid on a Paris apartment, where they were hiding — and planning another attack.
The money transfer to Abaaoud’s cousin isn’t Laachraoui’s only link to the Paris attacks. Traces of his DNA were found on the explosives used in the Paris bombings, French sources said.
And Belgian prosecutors said Laachraoui’s DNA was also found at a Brussels apartment where authorities found bomb making equipment in December.
Laachraoui may have acquired his bomb making skills in Syria in 2013.
It’s not clear when Laachraoui came back to Europe. But in September, he traveled through Austria and Hungary with Saleh Abdeslam — the only surviving suspect from the Paris attacks. Abdeslam was arrested at a Brussels-area apartment last Friday.
It’s unclear why a skilled bomb maker may have chosen to carry out a suicide bombing rather than continue making explosives.
“The level of skill to make these bombs, it requires many, many hours of preparation,” CNN national security analyst Perter Bergen said.
“If you have somebody who’s at the center of the cell who’s building these bombs who’s chosen to die, that would be a big victory for law enforcement.”
Ibrahim El Bakraoui: Convicted of shooting at police
Ibrahim El Bakraoui, 29, is the man shown in the middle of the airport photograph. He was the brother of Khalid El Bakraoui, who blew himself up at the Brussels subway station.
Ibrahim El Bakraoui left a will indicating he “needs to rush” and “no longer feels safe, Belgian federal prosecutor Frederic Van Leeuw said.
Police found the will on a computer in a trash can in Schaerbeek.
In 2010, a Brussels court sentenced Ibrahim El Bakraoui to nine years in prison for opening fire on police officers with a Kalashnikov rifle during a robbery, according to Belgian public broadcaster RTBF and CNN affiliate RTL.
The Turkish President’s office said authorities captured Ibrahim El Bakraoui in July 2015 and flagged him to Belgian authorities soon afterward.
“On July 20, the Belgian authorities responded to our note saying this person has a criminal record, but we could not determine links to a terrorism,” a senior Turkish official said.
“At this point, in the absence of an extradition request or an Interpol notice, an EU citizen has the right to be returned anywhere within the EU. So he was deported to Holland.”
A Dutch Ministry of Security and Justice spokeswoman said she could not confirm Ibrahim El Bakraoui’s extradition but said Dutch officials are investigating.
The Belgian federal prosecutor said the El Bakraoui brothers “had lengthy criminal records but (were) not linked to terrorism.”
Khalid El Bakraoui: The suspected subway bomber
Khalid El Bakraoui does not appear in the surveillance photo. He is accused of carrying out the attack at the metro station.
Interpol had a standing “red notice” for Khalid El Bakraoui, saying Belgian authorities wanted him in connection with terrorism. It’s unclear when Interpol issued the notice; Interpol did not respond immediately to CNN requests for more information.
Like Laachraoui, Khalid El Bakraoui and his brother have been accused of having clear ties to the Paris attacks.
According to a Belgian security source, Khalid El Bakraoui rented an apartment in the Forest district of Brussels that Abdeslam used as a hideout before his capture.
Man in the black hat: The mystery suspect
Police are scrambling to find the unidentified man seen next to Laachraoui and Ibrahim El Bakraoui in the airport surveillance footage.
Belgium’s interior minister said that man placed a bomb at the airport and left.
While two explosives went off within 37 seconds of each other shortly before 8 a.m., this third bomb — described as the “heaviest” by Van Leeuw — did not. Authorities later detonated the device in a controlled explosion.
A possible fifth suspect
On Wednesday morning, Belgian and French media reported the possibility of a fifth suspect involved in the Brussels attacks.
“Reportedly, there is a second (suspect) in the metro attacks,” Belgian public broadcaster RTFB said.
RTFB said a man was spotted on surveillance footage holding a large bag at the subway station. His identity is not known.
But the reports of a second subway suspect are preliminary, and officials are seeking more information.
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