The bulk of the evidence was already analyzed and digitized and stored on a computer server not on the site, he said. But justice officials say it is not clear if the courts will accept digitized evidence alone if the physical evidence is destroyed.

According to the Brussels prosecutor’s office, it is up to a judge to determine whether a digital copy of a lost piece of physical evidence has the same value. “So far this has never posed any problems,” said Ine Van Wymersch, a spokeswoman for the Brussels prosecutor’s office, which is handling the criminal investigation of the arson.

But Thierry Werts, a spokesman for the federal prosecutor, acknowledged that the crime lab has “thousands and thousands of forensic evidence pieces” from terrorism investigations, including from the Paris network. “If they’re completely destroyed, then that can be troublesome for all of our cases that are concerned,” he said. “If they’re only in part destroyed, then we’ll have to redo the analysis.”

Mr. Courtoy pointed to a number of terrorism cases that could potentially be affected. One involves his client Mehdi Nemmouche, who has been charged in an attack on the Jewish Museum in Brussels in 2014. His DNA was found at the museum, the authorities say, and was stored at the lab.

Another is that of Mohamed Abrini, the so-called man in the hat at the airport bombing, who is suspected of having participated in the Paris and Brussels attacks. His DNA was found in most of the hide-outs in Brussels, the authorities say.

And then there is the case against Yassine Atar, a suspected member of the Brussels terrorist network who authorities say helped hide Salah Abdeslam, a suspect in the Paris attacks who was captured after an intensive three-month manhunt. Nitrogen that could be used in making bombs was found on Mr. Atar’s hair and his beard, the authorities say, samples of which were sent to the lab for analysis.

Mr. Abdeslam was extradited to France this year, and his case may not be affected by what was lost in the fire because he will be prosecuted there, and much of the evidence was gathered in France. Still, Mr. Abdeslam’s Belgian lawyer, Sven Mary, expressed consternation at the lack of security at a government facility housing the forensic evidence of thousands of cases.