Najim Laachraoui has been identified as an accomplice of captured Paris attacks suspect Salah Abdeslam, Belgian prosecutors say.
The 24-year-old man is still on the run. A statement said he had been using false ID and that his DNA had been found in houses used by the suspected jihadist network.
Salah Abdeslam was captured in Brussels on March 18 and is still being interrogated.
However, Belgium’s federal prosecutor said he was “still far from solving the puzzle” of the Paris attacks.
“We have quite a few pieces of the puzzle and in the last few days several pieces have found their place,” Frederic Van Leeuw told journalists in Brussels.
ISIS said it was behind the Paris attacks.
The prosecutor’s statement said that Salah Abdeslam had traveled twice to the Hungarian capital Budapest, using a rental car in September 2015.
In the car were two other men, using fake Belgian identity cards with the names Samir Bouzid and Soufiane Kayal.
Soufiane Kayal has now been identified as Najim Laachraoui by DNA found at houses in the town of Auvelais and the Brussels district of Schaerbeek.
“The investigation showed that Soufiane Kayal can be identified as Najim Laachraoui, born on 18 May 1991 and who traveled to Syria in February 2013,” the statement said.
Belgian police said Samir Bouzid was “most probably” Mohamed Belkaid. He was killed by a police sniper in a raid on a flat outside Brussels on March 15.
The prosecutor appealed for public help in finding Najim Laachraoui.
Also still sought is Mohamed Abrini, who was filmed at petrol stations with Salah Abdeslam two days before the Paris attacks.
Salah Abdeslam’s lawyer, Sven Mary, meanwhile told Belgian radio that his client appeared “relieved” that the hunt was over.
He said: “Salah wants to co-operate and he’s done that since Saturday. Are we going to apply for the status [of informer]? He’s never said he wants to be an informer. I’ve never said that he’s asked for that.”
The lawyer added: “I have said that he can be of invaluable worth for different parties. He can give invaluable information for police as well as judges and lawyers.
“The statements of the French prosecutor… could ensure that Abdeslam is scared off.”
Sven Mary was referring to comments over the weekend by Paris prosecutor Francois Molins that Salah Abdeslam had admitted he wanted to blow himself up during the attacks, but then changed his mind.
Salah Abdeslam’s lawyer has filed a legal complaint against the prosecutor for violating secrecy.
Sven Mary also said he would continue to fight a transfer to France, but added: “Let’s be quite clear. He’s going to France – there is no single reason that he won’t go to France. It’ll be the investigating judge who decides when he goes.”
