PROSPECT — Authorities suspect someone late Friday night or early this morning set fire to a more than 100-year-old farmhouse on Main Street.

The homeowner was at work and no one was injured. The two-story house has moderate damage.

“We suspect arson,” said Chief Frank Wilson of the Prospect Volunteer Fire Department. “There was definitely evidence found at the scene.”

Wilson would not say what that was.

A passing motorist apparently spotted the fire at 413 Main St. and called 911, authorities said. The call was at 12:07 a.m.

Luck and the alert driver, Wilson said, possibly saved Bryan Frank’s house from going up in flames. The chief believed the likely arsonist had just lit the fire in the rear of the house when it was spotted.

“Another couple minutes,” Wilson said, “and it would have been really bad.”

More than two dozen firefighters from Prospect, Unionville and Portersville got there and found flames on the back wall.

Authorities estimated damage at $30,000 to $40,000. Frank has insurance on the house.

Frank is staying with his mother in Mount Chestnut.