Investigators are "far from solving the puzzle" of the Paris attacks, Belgian prosecutor Frédéric Van Leeuw told the media.
The inquiry is not over and other individuals have to be found, he said.
Belgian Foreign Minister Didier Reynders' said at the weekend that well over 30 people are believed to have been involved in the attacks and their aftermath.
Investigators don't know everything that Abdeslam did in the four months between the Paris attacks and his arrest and it remains to be seen whether he will tell his interrogators, the prosecutors added.
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The delay in catching Europe's most wanted man has been criticised in France, but French prosecutor François Molins was at pains to hail "very close" cooperation in the inquiry and in three others - Mehdi Nemmouche's attack on the Brussels Jewish museum, the attack on a Thalys train from Paris to Brussels and alleged terror plotters killed or arrested in the Belgian town of Verviers in January.
Earlier police announced they had identified one of Abdeslam's accomplices, previously known under the false name of Soufiane Kayal, as Najim Laachraoui, a 24-year-old who went to Syria in February 2013.