Another person seen carrying a large bag at the station also may have played a role in the subway attack.
And perhaps most puzzling, a man suspected of being a skilled ISIS bomb-maker gave his life while another terrorist simply planted a bomb and left.
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 two suspects believed to be alive and on the run.
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.
On Monday, the day before he died in the airport bombing, 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 those attacks, not only as the likely bomb-maker, but also as a coordinator of the violence.
In the days leading up to the assaults on the French capital, 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.
Both Abaaoud and his cousin were later killed in a raid on a Paris apartment, where they were hiding -- and apparently 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.
His younger brother told reporters that before he left for Syria, Laachraoui was a normal brother -- kind and intelligent.
He didn't tell his family ahead of time that he was leaving, according to Mourad Laachraoui.
The older Laachraoui was a soccer player and practiced taekwondo as a child.
Mourad Laachraoui said he didn't recognize his brother when he saw images of him on TV. He had no idea he was back.
It's not clear when Najim Laachraoui went back to Europe, but in September, he traveled through Austria and Hungary with Salah 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 would 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 Peter 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, was the man shown in the middle of the airport photograph. He was the brother of Khalid El Bakraoui, who blew himself up in the Brussels subway train.
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 the Schaerbeek district of Brussels.
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 Turkey 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."
The Dutch minister of security and justice offered more details in a letter to the lower house of Parliament.
He confirmed that Ibrahim El Bakraoui was deported to the Netherlands in July, but stressed that the reasons for the move were not known to Dutch authorities at the time.
"This was not a matter of an extradition or a return from detention or of an escorted return. El Bakraoui had a valid Belgian passport. On the 14th of July, 2015, El Bakraoui was not registered in the systems of either the Dutch or the international investigation and intelligence services. Therefore, when he arrived, on Schiphol (airport) there was no reason to take any action," the Minister said.
The Belgian federal prosecutor said the El Bakraoui brothers "had lengthy criminal records but (were) not linked to terrorism."
Meanwhile, a senior Turkish official told CNN: "We notified the Belgians and the Dutch of the deportation and the suspected links as a foreign fighter to Syria."
Ibrahim and Khalid El Bakraoui were included in a U.S. counterterrorism watch list, a U.S. official familiar with the matter confirmed to CNN on Friday. Ibrahim El Bakraoui was on the list before the Paris attacks last November for his suspected ties to terrorism.
Khalid El Bakraoui was added after the Paris attacks because of his alleged involvement in helping to pay for a safe house for some attackers.
Khalid El Bakraoui: The suspected subway bomber
Khalid El Bakraoui is accused of carrying out the attack on the subway train near a metro station.
Interpol had a standing "red notice" for him, saying Belgian authorities wanted him in connection with terrorism. It's unclear when Interpol issued the notice. The agency 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.
Unidentified suspect No. 1: The man in the black hat
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. Tuesday, this third bomb -- described as the "heaviest" by Van Leeuw -- did not. Authorities later detonated the device in a controlled explosion.
Unidentified suspect No. 2: Subway attacker?
Belgian authorities are seeking a second unidentified suspect from the metro bombing, a senior Belgian security source told CNN.
RTFB said a man was spotted on surveillance footage near Khalid El Bakraoui, holding a large bag at the subway station.
If that suspect is still alive, that means two suspects are still on the run.