"It doesn't seem to appear that way, but we're doing a full investigation on it like any other commercial structure fire," he said.
Clinton's Indiana campaign office had no immediate comment.
A volunteer firefighter driving past the building in a commercial district along U.S. 41 south of the city's downtown spotted flames on the roof shortly after midnight and warned those inside, Gardner said.
"They got out and a few seconds later the ceiling fell in where they were working and about five minutes later it was fully engulfed in the roof," he said.
The roof of the one-story building partially collapsed and a part of the front wall collapsed on a parked car, which then caught on fire. The adjoining office of a staffing services agency had water and smoke damage, but a fire wall held back the flames, Gardner said.
Yellow police tape blocked off the parking lot Friday morning as several investigators worked around the building.
Clinton is battling Illinois Sen. Barack Obama for the Democratic presidential nomination. Both candidates have set up offices throughout Indiana as they vie for the state's 72 delegates in the May 6 primary.
Former President Bill Clinton spoke Thursday night in nearby Vincennes and had a campaign visit set for Friday in the small city of Clinton about 20 miles north of Terre Haute. Obama was to speak Friday night at Terre Haute North High School.
