Arsonists set fire to Belgium's National Institute of Criminology in Brussels on Monday, causing an explosion but no casualties, a Brussels prosecutor said.
Five people have been arrested.
Ine Van Wymersch said there were no immediate indications that the fire at the institute, which was empty at the time, was a militant attack although nothing had been ruled out.
Europe has been on high alert after Islamic State attacks in Paris and Brussels over the past year.
“It was arson, deliberate arson at the laboratory of the federal police,” she said. “With a fire you get explosions, but it’s not that explosives were thrown inside or installed.”
“It is a path we are looking down,” she said, referring to the possibility of militant involvement. “But certainly not the first one we are thinking about. We are thinking more of deliberate arson by organised crime. We have no indications that it was terrorism,” she said.
The institute is linked to the Belgian ministry of justice and carries out forensic investigations in criminal cases, according to its website.
Belgian broadcaster RTL said that a car rammed through barriers at the centre at about 3am local time.