A spate of fires broke out along Interstate 90 near here Thursday, closing a stretch of the roadway to all but emergency traffic and threatening this Clark Fork River (search) valley community for a time, fire officials said.

The four-lane highway was closed Thursday evening from just west of Missoula (search) to St. Regis, just east of Lookout Pass on the Montana-Idaho border. The firefighting included use of water that helicopters with buckets scooped from the Clark Fork River, 20 miles of which was closed to public use as a safety precaution.

Charity Watt Levis, a spokeswoman for the state Transportation Department (search), said the 90-mile stretch of highway remained closed Friday while officials assessed hazards along the roadway.

"Visibility is still bad in the area and last night there was a lot of fire equipment on the roadway as well," Watt-Levis said. "And there was also some concern about a wooden retaining wall and we need to check for any damage to that."

She said the interstate would be reopened "when visibility is clear, if that retaining wall looks good and if the firefighting activities aren't a safety issue for travelers." On average, more than 7,000 vehicles pass through the St. Regis area on I-90 each day, Watt-Levis said.

With the interstate closed, traffic was being rerouted over Montana 200 — from the "Wye" west of Missoula to the Montana 135 exit at St. Regis. The Montana Fish, Wildlife and Parks Commission's order closing 20 miles of the Clark Fork River came Friday at the request of the U.S. Forest Service.

Officials said they were investigating arson or possibly a vehicle dragging something that emitted sparks because there was no natural way a dozen fires would break out at once. There were fires on both sides of the interstate.

"This absolutely wasn't an act of God," said fire spokesman Scott Waldron.

There were no injuries and minimal property damage, but Waldron said flames came just "inches away from burning" into Alberton.

Waldron said quick response by a massive crew of firefighters saved the town.

At times Thursday evening, a steady stream of helicopters swooped down on the Clark Fork, River, filling 500-gallon buckets and flying off to hot spots.

By dark, firefighters contained all but four of the fires, which had burned over a combined area of 250-300 acres.

In southwest Montana, the 300-acre Frog Pond fire near Philipsburg was considered 85 percent contained late Thursday. Officials said some of the nearly 300 people assigned were being moved into the Bitterroot Valley, along with equipment.