Belgian prosecutors now believe at least five people were involved in the attacks at Brussels airport and on a metro train. They're also investigating suspected links to the Belgium-based Paris attackers.
Brothers Khalid and Ibrahim El-Bakraoui
The brothers are both Belgian nationals who were well-known to authorities because of their previous criminal records for robbery. Although they were not thought to have been on terror watchlists before the attacks, Belgian authorities said they were hunting at least one of the brothers for links to the Paris attacks suspect Salah Abdeslam, who was arrested in Brussels last week. On Thursday, Abdeslam said through his lawyer that he didn't know of the plans to attack Brussels.
Twenty-nine-year-old Ibrahim El-Bakraoui was one of two suicide bombers at Brussels airport, where 11 people were killed. He was one of three suspects caught on CCTV inside the main terminal just before the twins blasts.
Turkey says Ibrahim was deported twice last year in July and August. In the first arrest, he was detained near the Syrian border and suspected of being a foreign militant fighter. He was sent to the Netherlands but was later freed due to a lack of evidence of any crime having been committed. He then returned to Turkey and was expelled again.
In Belgium, Ibrahim served four years of a 10-year sentence for armed robbery and was released in 2014. He skipped two probation appearances last June and was ordered back to jail in August, but his whereabouts were unknown.
Following the attacks, Belgian prosecutors revealed that Ibrahim had left a desperate "will" on a computer which he dumped in a trash can. In it, he said he felt "hunted" and added, "I don't know what to do."
In an apparent reference to Salah Abdeslam, the key suspect in the Paris massacre, he said: "I don't want to end up in a cell next to him."
An hour after the airport bombings, Khalid El-Bakraoui, who was 27, detonated his bomb inside Brussels' Maelbeek Metro station, killing 20 commuters.
In 2011, he was jailed for five years for a series of carjackings and was detained for possession of semi-automatic Kalashnikov rifles.
In December, an international arrest warrant was issued for Khalid after he rented an apartment in the Charleroi area of Brussels under a false name. The apartment was believed to have been used as a hideout by the Paris attackers, including main suspect Salah Abdeslam, who was arrested in Brussels last week.
Police think Khalid may have also provided ammunition for the Paris attacks.
Pictured alongside Ibrahim El-Bakraoui was the other Zaventem airport suicide bomber, Najim Laachraoui, who lived in the Schaerbeek district of Brussels. He detonated a suitcase bomb in the main terminal building. Both he and Ibrahim El-Bakraoui are pictured wearing a single black glove, which could have hidden trigger devices.
Known to Belgian security forces as a veteran Islamist fighter in Syria, he was confirmed as having stayed in the war-torn country during 2013. An international arrest warrant was issued for him in 2014.
A day before the Brussels attacks, police issued an arrest warrant for Laachraoui, saying he had traveled to Hungary last September with Paris attack suspect Salah Abdeslam.
The 24-year-old was suspected of making explosive belts for the November attacks in Paris. His DNA was found on at least two belts. He is believed to have gone on the run with Abdeslam before hiding in Brussels.
'Man in white'
A third suspect pictured in CCTV video wearing a white jacket, glasses and a black hat shortly before the attacks has yet to be identified and is believed to be on the run. Police said a bomb he was carrying failed to explode. An international manhunt is underway to find him.
French media reported that the mystery man is also linked to the Paris attacks.
The 'man in white' is thought to be a close friend of Mohamed Abrini, a Belgian of Moroccan origin, who is now the only outstanding cell member of the November attacks in France still at large.
Second Metro attacker
French and Belgian media cited unnamed sources saying a second attacker was seen on CCTV camera in the Brussels metro on Tuesday alongside Khalid El-Bakraoui.
Media reports say the man was carrying a large bag. He is not thought to have boarded the train with El-Bakraoui, who then detonated the bomb. It is unclear whether he was killed in the attack or is at large.