Prosecutor’s spokeswoman Ine Van Wymersch said a car broke through fences at about 2am yesterday morning.
The car was then rammed into a building where forensic tests are conducted, before a fire was started. Police could not say whether a bomb had exploded.
Ms Van Wymersch said the likely aim was to destroy “several files” inside.
The building is not yet safe to enter, Ms Van Wymersch said, so it was not possible to fully dismiss reports a bomb had exploded. The five people were arrested near the laboratories soon after the fire was reported, Ms Van Wymersch told a press conference.
“The location was not chosen randomly,” she said. “It’s an important site, that includes sensitive documents relating to current investigations.”
State broadcaster RTBF and other outlets had reported that a car drove through a security barrier at the site around 2am before an explosion caused significant damage to the facility on Brussels’ north side. Nobody was injured.
Investigators said the suspected arsonists set fire to a laboratory used to analyse DNA samples found at crime scenes.
She said any explosion heard by residents may have been caused by material being consumed in the fire.
There was no one at the site” in the northern Brussels suburb of Neder-Over-Heembeek, a spokesman for the prosecutor’s office said, refusing to comment on the cause except to say the explosion had a “criminal origin”.
The prosecutor has set up a “crisis centre”, the spokesman said.
Fire service spokesman Pierre Meys said the fire “was probably not accidental”.
“The explosion was extremely powerful,” said Mr Meys. “Windows of the lab were blown out dozens of metres away.”
He said around 30 firefighters were at the scene at around 3am fighting the blaze.
Belgium has been on high alert since coordinated suicide bomb attacks on the Brussels airport and subway killed 32 people on 22 March.
Belgium’s police and army have also been deployed in large numbers since suicide bombers attacked Paris last November, leaving 130 dead.
Many of the attackers had links to Belgium.
Tensions have been running high in Belgium in recent weeks amid a series of criminal knife and shooting attacks and two hoax anthrax attacks.