Prosecutor`s spokeswoman Ine Van Wymersch said a car broke through fences at about 02:00 (00:00 GMT).
The car was then rammed into a building where forensic tests are conducted, before a fire was started. Police could not confirm if 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."
Some 30 firefighters helped put out the fire at the National Institute of Criminology, which media said suffered "significant" damage but caused no casualties.
Forensic analysis linked to criminal cases is carried out at the site, but RTL report that it is not the only laboratory of its kind linked to the police.
Images submitted to RTL by nearby residents showed flames and heavy smoke rising into the night sky.
The independent institute, linked to Belgium`s federal justice body, is in Neder-Over-Heembeek, a suburb in the north of Brussels.
Bilal Hadfi, who blew himself up outside the Stade de France during last November`s Paris attacks, lived in the suburb.
Belgium`s terror alert level remains high since bomb attacks on Brussels airport and the city`s metro, claimed by so-called Islamic State, that killed 32 people in March.
Brussels crime lab fire: Five held over blaze `started to destroy evidence`
30 August 2016
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Five people have been arrested over a fire at crime laboratories in Brussels that officials believe may have been started to destroy forensic evidence.
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