Despite reports Thursday from firefighters and witnesses that a train “throwing out sparks” ignited the blaze, investigators said Friday morning that no cause had yet been determined.

“Any time we get any fire along a railroad track, especially multiple fires, obviously we have to look at it being a train,” said Matt Gomes, a regional forest ranger with the Maine Forest Service. “Our biggest emphasis is on possibly stopping a train that is spreading fire.”

But spokespeople for Pan Am Railways, which owns the tracks, and Amtrak, which runs passenger trains along the line, both said Friday that investigations cleared their trains, both of which passed over the tracks prior to the fire being reported just before 1:30 p.m.

Amtrak spokesman Craig Schultz said Friday morning that the southbound Downeaster passenger train that passed through the area after leaving Portland at 12:45 p.m. was inspected by Amtrak mechanics when it arrived at North Station in Boston.

“At this point our investigation showed that the train wasn’t involved,” Schultz said.

“To be quite frank, it would be very abnormal for us to have issues with Amtrak trains,” Gomes said.

A Maine forest ranger visited Dover, New Hampshire, on Thursday night to inspect the freight train that traveled through the area.

“That inspection came back and the locomotive was cleared,” Pan Am Railways Executive Vice President Cynthia Scarano said Friday. “The locomotive was fine and the [smoke] stacks were clean.”

But Gomes would not rule out a train as a cause of the fire. “A train can be legal and it can be maintained properly and it can be clean, and it can still start fires,” he said Friday.

On Thursday, Old Orchard Beach Fire Chief John Glass told the Bangor Daily News, “We’ve had some reports that one of the freight trains that came through town was throwing out sparks from beneath it. I don’t know if that was because the brakes were locked up or there was a malfunction or what the reason was, but there were a lot of sparks coming off one the freight trains and some of the cars, and that’s what started the little spot fires. It’s my understanding that it went from South Portland all the way to just about Biddeford.”

Glass said his department first received reports of small fires along the railroad tracks in the town around 1:20 p.m. Thursday. Firefighters from more than 20 towns battled the five-alarm blaze, which spread along train tracks from South Portland through Scarborough, Old Orchard Beach, Saco and Biddeford.

The Wagon Wheel RV Resort and Campground, at 3 Old Orchard Beach Road, sustained the worst damage along the line of fires, according to the chief.

The fire triggered a chain of propane tank explosions that destroyed at least 10 nearby campers.

No one suffered any major injuries in the damaging, sweeping blaze, although several firefighters suffered from exhaustion after working long hours in the heat, Glass said Thursday.

Among other possible causes for Thursday’s fires, Gomes said some stretches of train tracks are frequented by all-terrain vehicles and walkers.

But he added, “You have to take into consideration the amount of fire laid across five towns, and the speed with which it was laid down. That’s evidence itself.”