Two of the suicide bombers who carried out attacks in Brussels on Tuesday have been named as brothers Khalid and Brahim el-Bakraoui, Belgian nationals.
The federal prosecutor said Brahim was part of the attack at Zaventem airport that killed 11 people. Khalid struck at Maelbeek metro, where 20 people died.
Two other attackers at the airport have not yet been identified. One of them died, another is on the run.
Prosecutors say Brahim left a note in which he wrote of his desperation.
Belgium is observing three days of national mourning. The nation held a minute's silence at midday (11:00 GMT) on Wednesday. Belgium's king and queen have visited the airport and met some of the 300 people injured in the attacks. About 150 people remain in hospital, 61 in intensive care.
So-called Islamic State (IS) has said it was behind the attacks.
More about the attacks
Federal Prosecutor Frederic Van Leeuw said Brahim el-Bakraoui had been identified as the middle of three men in a CCTV image of the suspects of the airport attack.
The man on the left is believed to have died at the airport. The man on the right, wearing the hat, is thought to have fled the scene.
Unconfirmed reports in Belgian and French media suggest the man on the left is the wanted jihadist Najim Laachraoui.
Mr Van Leeuw told reporters that a taxi driver said he had picked up the three men from an address in the Schaerbeek area of Brussels. This apartment was raided later on Tuesday and bomb-making materials, including 15kg (33lb) of high explosive, were found.
A note from Brahim el-Bakraoui was found in a nearby rubbish bin. In it, he wrote: "I'm in a hurry. I don't know what to do anymore, they're looking for me everywhere. I'm not safe anymore. If I give myself up they'll put me in a cell."
Mr van Leeuw said the two brothers were known to police and had criminal records. They were identified by DNA records.
The RTBF broadcaster, quoting a police source, said that Khalid el-Bakraoui, 27, had used a false name to rent the flat in the Forest area of the Belgian capital where police killed a gunman in a shootout last week.
It was during that raid that police found a fingerprint of Salah Abdeslam, the main suspect in the Paris terror attacks of 13 November. He was arrested in a raid in Brussels last Friday and is due to appear before a pre-trial court on Wednesday.
Khalid el-Bakraoui appears on the Interpol website. It says that he is being sought for terrorist activities.
Turkey says Brahim was detained by Turkish officials on the border with Syria in June 2015. They deported him with the warning that he was a "foreign fighter" but the Belgian authorities let him go. Belgium has not yet responded to the claims.
The man on the right in the CCTV picture, who is being hunted, had a bag of detonators that were left behind. Mr Van Leeuw said the bag had contained "the biggest bomb", which later exploded "because it was so unstable" but did not harm anyone.
Najim Laachraoui was named earlier this week by police as a wanted accomplice of Abdeslam.
Analysts say Laachraoui is believed to be a key bomb maker, and French media say he also played a major role in the terror attacks in Paris.
Some Belgian media reported on Wednesday that he was the man arrested in Anderlecht area of the city, but Mr Van Leeuw denied the reports.
From the scene: Gavin Lee, BBC's Europe Reporter
There are still dozens of passengers being escorted out of the Brussels airport security zone, a day on from the biggest terrorist atrocity to hit Belgium. They were guests at the Sheraton Airport hotel.
Security services had advised spending the night at the hotel for their own safety, and intelligence officers have been visiting each room, interviewing guests and looking for anything suspicious. The hotel foyer became the immediate triage area for medical teams, bringing in the casualties and the bodies of those who died.
Now, dozens of military police and soldiers guard the airport entrance, with access only for forensic teams, investigators and airport staff. Several 'ghost flights' with no passengers on board set off this afternoon. Airlines are starting to move their aircrafts elsewhere, aware that 'business as usual' at Brussels airport could be a long way off.
The first of the victims to be named is Peruvian Adelma Tapia Ruiz, 37.
She had been at Zaventem airport with her Belgian husband and twin four-year-old daughters, who were unharmed, her brother told Peruvian radio.
World leaders have been reacting to the attacks. US President Barack Obama said fighting IS was his administration's "top priority", but added: "How do we do it in an intelligent way?"
France's Prime Minister Manuel Valls, on a visit to Belgium, warned that EU nations "will have to invest massively in their security system" in the coming years.
EU interior and justice ministers will hold a crisis meeting in Brussels on Thursday to discuss their response to the latest attacks.
Belgium has raised its terrorism alert to the highest level, and its international airport will remain closed on Thursday. Brussels Metro is due to close on Wednesday at 19:00 (18:00 GMT), operators say.