A Brussels apartment was probably used to make bombs for the Paris attacks and one of the plotters also hid out there after escaping a police dragnet, Belgian prosecutors have said.
The prosecutors said they found Salah Abdeslam's fingerprint in a search of the apartment on December 10, but would not say why they waited a month to announce it.
The search also turned up three suspected suicide belts, traces of the same explosive used in the November 13 attacks that killed 130 people and other material that could be used to manufacture bombs, according to the Belgian Federal Prosecutor's Office.
The third-floor apartment was probably used as a hideout after Abdeslam fled the attacks, federal prosecutor Eric Van der Sypt said.
Abdeslam, who is still at large, called for two friends to pick him up amid the bloodshed and chaos that night that left 130 people dead and hundreds injured.
"We found material to make explosives, we found traces of explosives and we found three belts. So you don't have to be Sherlock Holmes to make the right deduction," Mr Van der Sypt told the Associated Press.
Abdeslam is believed to have played a key logistical role in the Paris attacks. A French gendarme stopped him and his two friends in their car near the border but released them. The friends are among 10 people arrested in Belgium in connection with the attacks.
Authorities now believe Abdeslam returned to the apartment, was eventually picked up by someone else "and we lost trace", Mr Van der Sypt said.
The apartment in the Schaerbeek neighbourhood of Brussels had been rented under a false identity that may have been used by one of those who are now under arrest.
The three handmade belts discovered in the search at Rue Berge in Schaerbeek "could have been intended for the transport of explosives", the prosecutor's office said. Traces of the highly volatile TATP, which was packed into the suicide vests in November, as well as other material that could be used to manufacture explosives were also detected.
The November 13 attacks marked the height of a violent year for France that began with a January 7 assault on the offices of the Charlie Hebdo newspaper.
Paris was again jolted on Thursday when a man wearing a fake explosives vest and wielding a butcher's knife ran up to a police station and was shot dead by officers standing guard.
Paris prosecutor Francois Molins said investigators are unsure of the man's true identity.
Mr Molins told France-Inter radio that the assailant carried a paper marked with the Muslim declaration of faith, an emblem of the Islamic State group and his name, and gave his nationality as Tunisian. Mr Molins said he also had a phone with a German SIM card.
Stopped for a minor theft in 2013 in France's south, the man had identified himself as Ali Sallah and gave his nationality as Moroccan.
IS extremists have claimed responsibility for the January 2015 and November 13 attacks in Paris.