Suspect sought in alleged arson at Sen. Bernie Sanders' Vermont office: Police

A suspect is being sought in an alleged arson incident at Vermont Sen. Bernie Sanders' Burlington office, police said.

The fire was reported at 10:45 a.m. ET Friday in the vestibule between the elevator and the entrance door to Sanders' office, located on the third floor of 1 Church St., authorities said.

A man entered the vestibule and "sprayed an apparent accelerant" on the door to the senator's office, lit the accelerant and then fled, according to the Burlington Police Department.

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," the Burlington Police Department said in a statement.

The sprinkler system largely extinguished the fire, which caused "moderate" damage to the door to the senator's office, police said. The third floor and below floors sustained "significant" water damage, police said.

No injuries have been reported and Sanders was not present at the time, police said.

"The Burlington Fire Marshal's office has deemed this fire incendiary in nature," the Burlington Fire Department said in a press release.

A male suspect is being sought in the incident, Burlington police said while releasing surveillance stills of the unknown individual.

A motive is currently unknown, police said. The Vermont State Police and Burlington Police Department are conducting a joint investigation.

Kathryn Van Haste, Sanders' Vermont state director, said the office is "grateful" to the police and fire crews that responded.

"We are relieved that no one on our staff and, to our understanding, no one in the building was harmed," Van Haste said in a statement.

U.S. Capitol Police and the Senate Sergeant at Arms are working "in coordination" with local first responders, she said.

ABC News' Matt Foster contributed to this report.