SANFORD — The back building of the former Stenton Trust Mill complex on River Street was still smoldering Saturday, after a massive fire ripped through the structure Friday night.

Assistant Fire Chief Steve Cutter said there are still hot spots inside the five-story back building, which was gutted by the blaze. “This may smolder for a couple of days,” he said.

The fire brought 100 firefighters from 20 different departments to battle the blaze. While most departments left at daybreak Saturday, Sanford firefighters remained on the scene. They expected to stay at the former textile mill for days hosing down hot spots.

No one was injured in the blaze, and fire investigators were on the scene Saturday morning to look into the cause. City Manager Steven Buck said the property is being treated as a possible crime scene.

State Fire Marshal Joseph Thomas, in a telephone interview Saturday evening, said firefighters had been working all day to deal with rekindled fires at the mill, postponing an immediate probe into the cause of the blaze.

“It will probably be the first part of next week before we dig in and get an investigation underway,” Thomas said.

An infrared drone picked out trouble spots Friday night, and that helped firefighters contain the blaze, Cutter said. They spent the entire night spraying down the structure to try to prevent a collapse.

The destroyed back building is one of two structures connected by a one-story annex. The fire spread to a small portion of the identical front building, but the damage to that building was minimal.

The fire department earlier installed a red cross on the door to the front building warning fire department personnel not to enter. The Sanford building inspection department attached a notice dated March 27 saying that the front building was unsafe for human occupancy.

Multiple fires over the years have made both buildings unsafe to enter, city officials said.

Both structures are made of concrete and brick. The windows of the front building are riddled with graffiti and broken panes.

Curious citizens congregated at the base of River Street to take pictures of the smoking shell of the back building early Saturday.

Madeline Penney, who watched the flames Friday night from her nearby home, clicked away at the scene with her camera. She said her own home reeked of smoke from the blaze.

“The wind shifted and came into the house,” she said.

Paul Bock, owner of Black Sheep Archery on Cottage Street several blocks away, spent Saturday morning sweeping up the extinguished embers that rained down on his parking lot and the neighborhood of modest wooden homes north of the mill complex.

“There were charcoal briquettes the size of doughnuts dropping onto the roof,” Bock said.

The State Fire Marshal’s Office arrived on the scene before 9 a.m. ahead of the investigation into the cause of the fire. Buck said police are patrolling the area and city officials are looking to enclose the complex with fencing to keep people out.

The fire will also be investigated by the Sanford Police Department, Sanford Fire Department and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

The fire at the back building was reported about 6:50 p.m. Friday and spread quickly, with flames leaping from the windows and smoke filling the downtown.

It was unclear Saturday who is responsible for the complex. The last owner of record is Jonathan Morse of Gateway Properties LLC, whose address in Sanford tax records is listed as Reno, Nevada. He owned mill buildings in Biddeford before he left the area more than a decade ago, said Maura Herlihy, deputy mayor of Sanford.

Herlihy said the city came close at least once to finding a private partner to help redevelop the 294,000-square-foot complex, which was built in 1922 as a textile mill. The complex was considered an important piece of Sanford’s development plans for its vacant mill buildings.

But interested developers would drop out when it became clear that the back building was in terrible shape.

A redevelopment company, Boston Commons Investments LLC, paid $210,000 for the property in a foreclosure auction in 2009, then backed out of the deal because of the poor condition of the back building, Herlihy said.

Herlihy said the buildings attracted teenagers and vagrants.

“There is not one kid I haven’t talked to who didn’t go in there as an adventure,” Herlihy said.

Buck said it is unclear who will pay for the cleanup and demolition of the building.

The property may qualify for brownfield funds from the U.S. Environmental Protection Agency, Buck said. Like many buildings of its era, the complex contains asbestos and lead contamination.

Buck said there were no serious environmental problems from Friday night’s fire. The Maine Department of Environmental Protection was on site monitoring runoff. Buck said there were no concerns about hazardous materials in the smoke.

Staff Writer Peter McGuire contributed to this report.