The authorities in Vermont said they were searching for a man who started a fire outside Senator Bernie Sanders’s office in Burlington on Friday morning.

The unidentified man walked into the vestibule of Senator Sanders’s office and sprayed an “apparent accelerant” on the entrance door to the third-floor office, before lighting the accelerant and fleeing, the Burlington Police Department said in a statement. It was unclear exactly what the man sprayed on the door.

“A significant fire engulfed the door and part of the vestibule,” which prevented staff members who were working inside the office from exiting, the Police Department said.

The building’s sprinkler system activated, which mostly put out the fire before firefighters arrived around 10:45 a.m., the Police Department said.

The Burlington Fire Department said that the door to the senator’s office “sustained moderate” damage from the fire, and that the third floor of the building and the floors below it also had water damage.

No injuries were reported. Senator Sanders, independent of Vermont, was not at the office at the time of the fire, his office said in a statement.

Investigators with the Vermont State Police determined that the fire was an act of arson. The authorities had not concluded a motive.

The Police Department released a photo from surveillance footage of the man who started the fire. In the photo, he is wearing a black jacket, dark-colored pants, white sneakers and an orange beanie.

Kathryn Van Haste, the Vermont state director for Senator Sanders, said in a statement, “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.”

The United States Capitol Police and the Senate sergeant-at-arms were working with local authorities in Burlington investigating the fire, Ms. Van Haste said.

Mayor Emma Mulvaney-Stanak of Burlington said in a statement that she was “relieved to hear that everyone made it out safely.”