A Sanford firefighter sprays water into the mill that caught fire Friday in Sanford. The building is unstable so investigators haven't been able to enter it.

Smoke rises from flare-ups as firefighters continue to work Saturday morning in Sanford.

Sanford Fire Chief Steve Benotti talks to the media outside the mill that burned on Friday. The rear building is the one that burned, and officials said that the fire was set by three boys.

Paul Bock, co-owner of Black Sheep Archery on Cottage Street, sweeps up embers from the fire on Saturday morning. Bock said that at one point, donut sized embers were falling from the sky.

Firefighters from 20 communities work to extinguish a blaze at a mill building in Sanford on Friday. This photo was taken from a drone.

A red X marks the entrance to the front building of a vacant mill complex on River Street in Sanford.

Firefighters continue to work at the scene of a large fire at a vacant mill complex on River Street in Sanford on Saturday.

The fire at a vacant mill building in Sanford created intense heat.

Spruce Street saw plenty of visitors Saturday viewing the scene of fire at a vacant mill complex on River Street in Sanford.

A vacant mill building caught fire in Sanford on Friday evening resulting in a five-alarm fire.

Warren Royea, who lives across from the mill in Sanford, noticed the fire around 6:45 p.m. and notified a police officer who had just made a traffic stop nearby. The fire quickly engulfed the building.

Crews from 20 communities helped Sanford fight the blaze.

Warren Royea, who lives across from the mill, said the fire moved rapidly. “It didn’t take long at all. It spread in minutes.”

Smoke billowed from the vacant mill building Friday.

Firefighters from at least six communities helped fight the fire at a Sanford mill Friday.

The fire at a vacant Sanford mill on Friday sent smoke hundreds of feet into the air.

The neighborhood in Sanford where a mill burned Friday was covered in ash from the blaze.

Warren Royea, who lives across from the mill, said of the fire, “I heard it before I saw it.”

A major fire blazed through a mill in Sanford on Friday night.

Onlookers gather to watch as firefighters work to put out the five-alarm fire in a vacant mill on River Street in Sanford on Friday evening.

The fire at a vacant Sanford mill Friday drew a multitude of onlookers, many of whom posted videos and photos on social media.

Smoke continued to pour from the building as firefighters doused it with water.

Firefighters continue to douse a fire at a vacant mill in Sanford as night falls Friday.