Manassas City officials say that what may have started as a school prank was an act of arson.

A fire that started in a boys’ restroom at Grace E. Metz Middle School on March 20 caused an evacuation in which students spent an hour outside the building, said Patty Prince, a spokeswoman for the city police department.

After an investigation, the fire marshal’s office determined that pieces of paper had been set on fire in the restroom. The fire and smoke caused damage to stalls, some of which had to be replaced. Prince said the cost of cleanup, repairs and replacement fixtures in the bathroom would be $20,000. The damage would have been greater if the sprinklers in the school had been activated, but firefighters put out the flames with fire extinguishers and the sprinklers were not triggered.

Investigators think the fire was started by one or more students, Prince said, primarily because teachers and other adults in the school do not use the bathroom where the fire started.

After completing the investigation into the cause of the fire, the fire marshal’s office announced Wednesday that it was treating the incident as an arson case and asked members of the community to call with any information about the perpetrator or perpetrators. The announcement said that tipsters, who can leave information anonymously, may receive a reward of up to $1,000 for information that leads to the arrest of a perpetrator.

Prince said Thursday that many tips had come in. She would not say whether police had interviewed students or whether any students had witnessed the start of the fire.

Administrators at the school did not respond to a request for comment Thursday.

Tipsters can leave anonymous messages by calling 703-330-0330 e-mailing firemarshal@