The brothers identified as the Isis (Islamic State) bombers who attacked Brussels had a record of gang-related crimes in the Belgian capital and had been in touch with terrorists who struck Paris in November.

Surveillance footage showed two men dressed in black and a third in light-coloured clothing pushing luggage carts through Zaventem Airport before the explosions. Belgian federal prosecutor Frederic Van Leeuw said that the man in the middle of the trio, Ibrahim El Bakraoui, detonated explosives at the terminal and had been identified from his fingerprints. His brother Khalid attacked the Maelbeek metro station, Van Leeuw said. The bombings killed 31 people and injured 270.

Khalid, 27, and Ibrahim, 29, had criminal records, including for armed robbery and carjackings, but weren't wanted in relation to suspected terrorism offenses until security services raided an apartment in the Forest borough of Brussels last week, according to state broadcaster RTBF.

Khalid is thought to have rented the property under a false name, according to local media. There, police found a Kalashnikov rifle, an Isis flag and a book about radical Islam. They also recovered DNA from Salah Abdeslam, thought to be the sole surviving participant of the massacre in the French capital.

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Abdeslam was arrested by Belgian police on March 19 after four months on the run. A member of the Paris attacks cell, Mohamed Belkaïd, was shot dead by a police sniper at the flat. The El Bakraoui brothers are believed to have fled.

It's not uncommon for brothers to carry out such attacks together. In fact, as security and intelligence agencies step up their monitoring of potential extremists, jihadist groups are increasingly looking to recruit family members, a move that makes it harder to detect and prevent radicalisation, according to a paper published last month by the Combating Terrorism Centre at West Point.

"Kinship recruitment, which is difficult for security agencies to observe, is facilitated by several psychological mechanisms that bind individuals together on the path to extremism," the report's author, Mohammed Hafez, wrote. "Importantly, it deters ambivalent recruits from defecting to the authorities for fear of damaging their own valued relationships."

The bombing of the Boston marathon in 2013 was carried out by the Tsarnaev brothers, while Cherif and Said Kouachi attacked the offices of the Charlie Hebdo satirical magazine in Paris last year. A husband-and-wife team was responsible for the San Bernardino mass shooting in December.

Tamerlan, left, and Dzhokhar Tsarnaev, brothers who planted bombs at the finish line of the Boston Marathon on April 15, 2013. Photo / AP
Tamerlan, left, and Dzhokhar Tsarnaev, brothers who planted bombs at the finish line of the Boston Marathon on April 15, 2013. Photo / AP

Ibrahim El Bakraoui was convicted in 2010 of firing a Kalashnikov rifle at police during an armed robbery, according to reports by RTBF. He wounded one of the officers and was sentenced to nine years in prison. It's not clear when he was released. Khalid was sentenced to five years on probation in 2011 for carjackings, the broadcaster said.

Neither Ibrahim nor his brother appear to have been active on social media, at least not under those names, and no clear picture of their path to radicalisation has emerged.

Van Leeuw, the Belgian prosecutor, said in the news conference police had recovered a message from a computer found in a garbage bin that suggested Ibrahim launched the airport attack as he knew authorities were closing in on him.

"I do not know what to do. I am on the run, people are looking for me everywhere. If I give myself up I will end up in a prison cell next to him," Ibrahim says in an apparent reference to Abdeslam.