Belgium's federal prosecutor confirmed on Friday that Najim Laachraoui was one of the two suicide bombers who struck Brussels airport this week in a series of attacks which left 31 people dead.

The prosecutor also linked Laachraoui to November's Paris carnage in which 130 people died, saying his DNA was found on a suicide vest and a piece of cloth discovered at the Bataclan concert hall where 90 people were killed.

Police also found his DNA on a bomb at the Stade de France, a statement said.

CCTV footage showed three bombers at Brussels airport on Tuesday, with Laachraoui and Ibrahim El Bakraoui now being named as the two who blew themselves up there.

A third man on the right of the CCTV frame, wearing a light coloured coat and a hat, fled the scene, apparently after his bomb, said to be the largest of the three, failed to detonate.

The police have launched a massive search for this third man.

The prosecutor's office also confirmed that Laachraoui had previously used the false identity of Soufiane Kayal.

Key Paris attacks suspect Salah Abdeslam went to the Hungarian capital Budapest in September and on his return was stopped by police on the Austrian border with two men - Soufiane Kayal, as he was then known, and Samir Bouzid.

Abdeslam was arrested in central Brussels last Friday.

Laachraoui, 24, went to Syria in early 2013.

He was a good student with an immaculate disciplinary record at a Brussels Catholic school, and friends and family remember a bright easy-going kid who liked to play frisbee and football.

"I don't understand how anyone can be brainwashed so quickly," Brice Vanhee, a college friend from his first year electro-mechanical engineering course in 2012, said on Facebook.

"How can you switch sides and blow yourself up when you used to play frisbee tournaments every weekend? I don't get it!"

A picture on Vanhee's page shows a group of eight students including Laachraoui smiling at the camera.