BRUSSELS (AFP) - Long before blowing himself up in a crowd of innocent people, leaving behind a desperate will on his computer, suicide bomber Ibrahim El Bakraoui and his brother Khalid lived the lives of the classic Brussels hoodlum.

Carjackings, robberies and shoot-outs with police were just some of the convictions collected by the brothers who took part in the deadly train and airport attacks in Brussels.

Baby-faced Ibrahim, 30, one of two suicide bombers at the airport, had been handed a nine-year sentence in 2010 after a gunfight with police, according to local media.

He took part in a bungled robbery at a Western Union office in which a police officer was injured in the leg.

Media reports said he was stationed as a lookout in the attempted theft when police surprised the six assailants.

But the picture that emerges from a confused and scared message left on an abandoned computer is a very different image from that of Brussels gangster.

"Hunted everywhere... no longer safe," Ibrahim said, according to the federal prosecutor, adding that "I don't know what to do".

Meanwhile, brother Khalid, 27, who blew himself at the Maalbeek metro station in the heart of the European district in Brussels, is a convicted carjacker, receiving a five-year sentence 2011, according to daily La Derniere Heure.

Federal prosecutors revealed at a dramatic briefing on Wednesday that both brothers were Belgian citizens with convictions "not linked to terrorism."

The pair burst into the public eye on March 15 when police raided an apartment in the Forest district of Brussels, as part of the investigation into the Paris attacks.

Belgian media reports said a joint squad of Belgian and French police approached the property because it was rented under a false name used by Khalid to secure a hideout months earlier for the Paris attackers.

Belgium filed an Interpol search for Khalid, who is suspected of renting other properties used to prepare the November tragedy in Paris, including one in the southern Belgian city of Charleroi from where ringleader Abdelhamid Abaaoud set off to spearhead the assault.

The police last week fully expected to find an abandoned property, as utilities in the Brussels apartment had been shut off for weeks.

Instead they came under heavy gunfire, with one assailant killed and two men fleeing - including, it is thought, Salah Abdeslam, the top Paris fugitive who would be arrested three days later.

The brothers are not believed to have been present but their connection to Islamic State jihadism was established.

Another possible participant in the Brussels attack is suspected Islamic State in Iraq and Syria (ISIS) militant Najim Laachraoui, who could be the man dressed in a light jacket seen in CCTV images at the airport.

Moroccan-born Laachraoui is suspected of being the bombmaker for the Paris attacks that killed 130 people.

Investigators have found traces of his DNA on explosives used in the Paris bomb and gun assaults, including at the Bataclan rock venue where 80 people died.

DNA traces were also found in a rural Belgian hideout used on the eve of the Paris attacks, as well as in a suspected bomb factory in the Schaerbeek district of Brussels.

Laachraoui grew up in the multi-cultural Schaerbeek neighbourhood of Brussels and has been wanted by police in connection with the Paris attacks since Dec 4, though by a false identity that was only unmasked last week.

Laachraoui attended Schaerbeek's Sainte-Famille Catholic school for six years where a school official told AFP he was a "typical" student.

He received his baccalaureat, the equivalent of a high-school diploma, there in 2009.

A newsletter posted on the school's website indicated that he studied electronics, but the official would not confirm this.

Laachraoui left for Syria in September 2013 in one of the first waves of jihadists to leave Belgium for Syria, where he fought under the Islamic State nom de guerre Abu Idriss, according to media reports.

In February, a Belgian court convicted Laachraoui in absentia for his involvement with ISIS.

He is known to have returned to Europe in September when he was checked by police under a false identity in a Mercedes driven by Abdeslam, who now sits behind bars in Belgium.

Also in the car was the Algerian Mohamed Belkaid, 35, shot dead during the March 15 search and gun battle in Brussels that eventually led to Abdeslam's capture.