'So it's an act that could be linked to (destroying) several files, ' Van Wymersch said. 'I can not confirm that there was any bomb'.
Local media has reported that one or more people in the vehicle detonated a bomb, causing the blast, but Belgian police have not yet confirmed what happened.
Earlier in the day, media reports emerged claiming that a auto rammed through the fence surrounding the building of the institute, located in northern Brussels, after which one or more attackers exploded a bomb at the site. Those arrested were released after questioning.
The fire caused a blast but no casualties.
Ms Van Wymersch said the location was not chosen randomly and the lab contains sensitive information being used in ongoing investigations.
The institute is part of Belgium's federal justice system.
State broadcaster RTBF and other outlets had reported that a auto drove through a security barrier at the site followed by an explosion.
Among the institute's tasks is carrying out forensic analysis for criminal cases.
The fire and damage were hampering the efforts of investigators to enter the scene, and she said it would take some time for them establish exactly what had been destroyed.
He said about 30 firefighters were at the scene at around 3:00am (0100 GMT) fighting the blaze.
Fire service spokesman Pierre Meys told Agence France-Presse the "extremely powerful" blast "was probably not accidental".
Europe has been on high alert after ISIS attacks in Paris and Brussels over the past year. Abdeslam was captured in Brussels on March 18.