A 113-year-old wooden bridge in a swampy swath of northern Minnesota was set ablaze and severely damaged, and authorities are searching for whoever torched the historic rail crossing.

Hikers and snowmobilers who often crossed the historic bridge in Blackduck, Minn., are mourning the loss of the key piece of the area's trail system and wondering whether and when it will be rebuilt.

"It's a pretty important little bridge," said Ray Guthrie, owner of the Tepee-Tonka Resort on Blackduck Lake. "The community would like to have it back, if it could, but how do you replace a 100-year-old bridge?"

Fire crews and other first responders answered a 911 call shortly after 5:30 a.m. Monday and found the bridge "fully engulfed" in flames on the east side of Blackduck, according to the Police Department. By the time the blaze was extinguished, nearly 30 percent of the 700-foot-long bridge was destroyed. State, county and city fire investigators "revealed the cause to be arson," according to a statement from police.

The bridge, once a critical rail link, had lived on for recreational use but is now off-limits.

"This act of vandalism … has effectively destroyed a public landmark and the use of the bridge by those who are enjoying recreational activities in the Blackduck area," the statement added.

The bridge's burning has been "quite the buzz" at Funkley Bar and Lounge, just north of Blackduck on Hwy. 71, said owner Carrie Erickson. Local kids often carved their names into the bridge's underside, she said.

"They've done that for years," Erickson said Tuesday. "And I thought, oh, all them little names are gone."

Customers are upset by word that arson caused the fire, said Erickson, who has lived in the area for 17 years. "It's sort of scary thinking that there's someone out there that's going to do something like that."

The Blackduck Trestle, also known as the M & I Trestle, was built during 1901-02 for use by the Northern Pacific subsidiary M & I Railroad. In February 2014, long after the trains stopped rolling, the bridge was added to the National Register of Historic Places. Police said it was valued at $500,000.

The span, the longest on the former M & I line, traversed Coburn Creek and the surrounding marsh while providing a rail link back in the day between Bemidji and International Falls, according to the register. The crisscross style of supports was a common design in rail bridge construction.

The bridge's description in the register also noted it was one of the longest of its kind still standing in the state.

City Administrator Christina Regas went out Monday to see the damage to the bridge, which she has used many times while snowmobiling.

"All I could smell was charred timber," Regas said. Unless you stood in the middle of it, she added, "you can't really get a feeling of how much has been damaged."

Regas said the assault on the historic yet functioning landmark has shaken many in and around Blackduck.

In 1996, the Minnesota Department of Transportation assumed ownership of the bridge after the railroad ceased operations. MnDOT was in the process of turning the span over to the state Department of Natural Resources.

Anyone with information about this case is urged to call police at 218-333-9111.

Staff writer Jenna Ross contributed to this report.

Paul Walsh • 612-673-4482