A fire at Sen. Bernie Sanders’ Vermont office is being investigated as arson, the Burlington Fire Department said Friday.

Firefighters were dispatched Friday morning to the building that houses the independent senator’s only congressional office in the state. They found “a fire in the vestibule between the elevator and the entrance door to [Sanders’] office,” the fire department said in a news release.

Police in Burlington said “an unknown male subject entered the vestibule” of Sanders’ office and then “sprayed an apparent accelerant on the entrance door.”

“He then lit the accelerant and fled,” the police statement said. It also said that a “significant fire engulfed the door and part of the vestibule, impeding the egress of staff members who were working in the office and endangering their lives.”

There were no reported injuries. The door to Sanders’ office was damaged by the fire and other parts of the building had water damage.

The suspect has not been apprehended, and there is no known motive, according to police.

“We are grateful to the Burlington Fire and Police Departments who responded immediately today to a fire incident that took place in our office building,” said Sanders’ state director Kathryn Van Haste in a statement. “We are relieved that no one on our staff and, to our understanding, no one in the building was harmed.”

Van Haste said that U.S. Capitol Police and the Senate Sergeant at Arms are coordinating with local first responders.

The Burlington Fire Marshal’s office, Burlington Police Department and Vermont State Police are conducting a joint investigation.

A spokesperson for the state police did not immediately respond to a request for comment Friday night.

The suspected arson comes as Capitol Police recorded an uptick in threat assessment cases last year. Police investigated more than 8,000 cases — about 500 more than the previous year — including cases involving “concerning statements and direct threats,” according to a January release.

Sanders, 82, is in his third term as senator. He previously served in the House for 16 years, and before that was mayor of Burlington, where he lives with his wife.