Attackers rammed a car through the gates of Belgium's crime institute and set fire to a lab containing crime scene samples, apparently in an effort to destroy evidence.
Residents heard at least one explosion as the blaze erupted in a north Brussels suburb and Belgian media reported a bomb attack, but investigators said the noise was probably materials going up in flames.
Prosecutors said five people were detained for questioning and released without charge following the incident at about 2am on Monday in Neder-Over-Heembeek. No one was injured.
"It's probably not terrorism. It's a criminal act," said Ine Van Wymersch, a spokeswoman for the Brussels prosecutor's office.
"I cannot confirm that there was any bomb."
The forensic facility assists Belgium's justice authorities in carrying out their investigations and the lab contains DNA samples found at crime scenes.
"The location was not chosen randomly," said Ms Van Wymersch.
She said the lab contains sensitive information being used in ongoing investigations.
The fire and damage hampered the efforts of investigators to enter the scene, and she said it would take some time to establish exactly what had been destroyed.
"The laboratory does thousands of analyses each year, so we don't know what damage has been done yet," said Ms Van Wymersch.
"It is obvious that several individuals would have an interest in making elements in their justice file disappear."
Belgium has been on high alert since suicide bombings at the Brussels airport and subway killed 32 people on March 22, and security forces remain on standby for another attack.
The police and army have 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 also been running high in recent weeks amid a series of criminal knife and shooting attacks and two hoax anthrax attacks.
One person was killed and at least four were injured on Friday in an accidental explosion at a sports centre near the French border.
Belgian prime minister Charles Michel announced new plans to ease the load on the security services at the weekend.
They include creating a new force to protect key buildings and using more private security firms for concerts or sports events.