A bridge on the Lake Wobegon Trail that was purposely set on fire reopened Monday, the same day that the Minnesota Arson Reward Project announced it was offering a $5,000 reward for information that leads to an arrest and conviction of those responsible.

Inspectors from Stearns County and the Minnesota Department of Natural Resources on Monday declared the bridge at Schwinghammer Lake structurally sound after a fire Saturday night damaged some of its wooden planks, the Stearns County Sheriff's Office said.

That came as good news and a relief for Cliff Borgerding, president of the Lake Wobegon Trails Association. The trail is the site of two marathons in the coming weeks and is popular with bicyclists, in-line skaters and walkers.

Had the bridge been declared impassable, "that would have been a huge hit to the trail," Borgerding said.

Quick action by bicyclist and trail advocate Bobby Youngdahl may have kept damage to a minimum. He was biking from St. Cloud to Albany to meet a friend when he crossed the bridge at mile marker 94 around 5:15 p.m. He smelled a "wisp of smoke" and at first thought maybe somebody was smoking under the west end of the bridge. He continued on a bit, but then circled back to be sure that nothing was amiss.

That is when Youngdahl saw smoke and flames through the planks. He called 911. Within a couple minutes, the windwhipped flames were shooting out from beneath the bridge.

"I heard the crack of flames and smelled the creosote," he said on Monday. "I'm glad I didn't keep riding on."

Officials on Monday ruled the cause of the fire as arson and that an accelerant had been used, said Stearns County Sheriff's Deputy Bruce Bechtold.

Firefighters from Avon, Albany and Freeport spent five hours dousing the flames.

"Thank you so much Bobby for your help getting this reported as quickly as you did … you definitely saved the bridge from even more severe damage," a posting on the Lake Wobegon Trails Facebook page said.

Damage was limited to the wooden planks. The 216-foot bridge is supported by large steel beams that are strong enough to hold trains, and they were not damaged. The rest of the bridge, which is concrete and asphalt, is in fine shape as well, the Sheriff's Office said.

The Arson Tip Hotline is at 1-800-723-2020. Tipsters can remain anonymous.