PORTLAND, Oregon (Reuters) - A propane gas tank rigged up as a homemade bomb exploded in flames outside a courthouse before dawn on Wednesday in the southwestern Oregon town of Medford. No one was hurt and damage was minor, police said.

Details of the blast were sketchy, but Medford police Lieutenant Mike Budreau said officers were called to the Jackson County courthouse at about 4:30 a.m. local time (7:30 a.m. EST) to investigate reports of an explosion.

They found a damaged seven-gallon propane tank ablaze, as well as broken glass from shattered windows. Damage to the brick building appeared to be mostly confined to the exterior, Budreau said.

"We believe the suspect tried to detonate the propane tank with an improvised explosive device (that) damaged the propane tank," he said, adding that the tank caught fire but remained mostly intact.

No arrests have been made, and the motive behind the firebombing was unknown, he said.

"We're not sure if this is targeted towards a prosecution that was going to happen or has happened or whether it's something else," he said. He said the investigation was "wide open at this point."

According to an account published by the Medford Mail Tribune newspaper, police reported that an unidentified male in dark clothing was seen several blocks from the blast and ran away from an officer when approached.

Oregon state police and federal agents were assisting in the investigation, authorities said.

(Reporting by Teresa Carson in Portland; Additional reporting by Chris Francescani in New York; Writing by Steve Gorman; Editing by Cynthia Johnston and Leslie Gevirtz)