Investigators have made progress in piecing together who carried out this week's terrorist attacks in Brussels and why, but some questions still remain open.

WHAT WE KNOW:

- The attacks were carried out on the morning of March 22 by at least four people. Two suicide bombers blew themselves up at the Belgian capital's international airport, while a third dropped off an explosive device and then left. A fourth attacker blew himself up in an underground train at the Maelbeek station, near the seat in Brussels of the European Union's main institutions.

- The three dead suicide bombers have been identified. Two of them are Brussels-born brothers who had criminal records in Belgium, but not for terrorist offences: Ibrahim El Bakraoui, 29, and Khalid El Bakraoui, 27. Ibrahim was one of the suicide bombers at the airport, while Khalid was at the Maelbeek station. Najim Laachraoui, a 24-year-old Belgian national who had long been sought in connection with last year's Paris terrorist attacks, was identified on Friday as the second suicide bomber who died in the airport attack.

- The attacks have left at least 35 dead, in addition to the three attackers. Thirty-one victims died at the crime scenes, of whom 28 had been identified by Monday. A further four people have died in hospital, the Belgian Health Ministry said Monday. In addition to Belgians, foreigners of eight different nationalities also died, according to the Belgian crisis centre.

- The attacks also left around 340 people injured. As of Monday afternoon, 96 were still in hospital, 55 of them in intensive care and 32 being treated for severe burns, the Health Ministry announced. Nineteen foreign nationalities have been registered among the injured victims so far, according to the Belgian crisis centre.

- The Islamic State extremist group has claimed responsibility for the attack, accusing Belgium of "attacking Islam and its people." Belgium is part of a US-led coalition that has carried out airstrikes against Islamic State in Syria and Iraq.

- The investigative findings communicated by Belgian prosecutors increasingly point to a link between the Brussels suicide bombings and last November's terrorist attacks in the French capital Paris. Laachraoui and Khalid El Bakraoui are suspected of having rented real estate in Belgium allegedly used by the Paris attackers. Traces of Laachraoui's DNA were also found on explosive vests used in Paris.

WHAT WE DON'T KNOW:

- The identity of the third airport attacker, who is still thought to be at large, remains unknown. Belgian media had reported that it was Faycal Cheffou, who was taken into custody on Thursday. On Saturday, prosecutors announced that a man they identified only as Faycal C had been charged with terrorist murder, attempted terrorist murder and participation in the activities of a terrorist group. But Faycal C was released on Monday, after investigators found no evidence to support the charges against him. Police released video surveillance footage of the fugitive suspect on Monday, after previously issuing photos and asking the public to help identify him.

- There have been media reports that a second attacker was at the Maelbeek metro station, but this has not been confirmed by Belgian officials. The state broadcaster RTBF said it was unclear whether the unidentified man, who was captured on surveillance camera carrying a large bag, was still alive.

- It still remains to be determined if other people helped the Brussels attackers, for instance with logistical assistance. German police said Friday that they had arrested two men in connection with the Brussels attacks. One of them is said to have had messages on his mobile phone linking him to the suicide bombings.

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