EVERETT — Neighbors used ladders to rescue people from the upper stories of a burning apartment building in south Everett on Thursday night.

Within 10 minutes of the first 911 call, people were helping each other escape, leaving the doors to their homes open as they fled walls of flames.

Not everyone made it out. One person was killed, an adult. The person’s age and gender were still undetermined as of Friday morning, said Eric Hicks, the assistant fire marshal for Everett. The Snohomish County Medical Examiner’s Office collected the body and will work to confirm the identity.

The three-alarm fire at the Bluffs apartments at 2 West Casino Road is believed to be the largest blaze in Everett in more than three years.

A dozen people were hospitalized, including children. A 57-year-old woman had critical injuries, and three others were seriously hurt, Hicks said.

In all, 15 people received medical treatment Thursday night for smoke inhalation, burns or injuries received while jumping or being dropped from upper-story windows, Hicks said. The ages of those injured ranged from 5 to 60.

Crews on Friday had not received an update on the victims’ conditions, he said.

Witnesses reported seeing people jumping from the second and third floors, and said a man ran up with a ladder he apparently pulled from the rack on his work van.

Multiple ladders that had been used by neighbors could be seen about the property Friday.

There were few onlookers on the cold, clear morning, where steam could be seen rising from the heat of the burned-out roof.

People who live in the complex gathered at the leasing office for a briefing from the fire department. Some wore blankets over their pajamas as they headed inside.

The building had no fire sprinkler system, and the flames spread through the attic, Hicks said.

The fire followed a V-shaped pattern, radiating out from the second-floor apartment where it apparently started in a mattress.

“So far there is no reason to suspect it was intentionally set, but we’re still investigating,” Hicks said. More than 100 people were displaced. Of those, 15 stayed Thursday night in a shelter set up at a nearby church, said Chuck Morrison, director of the Snohomish County branch of the American Red Cross. His team on Friday was trying to get in touch with everyone who needed assistance.

“We will assume it will be very difficult to find housing over the weekend, so we’ll be open at least through the weekend and help folks from there,” he said.

Firefighters on Friday also were allowing people inside to retrieve medication and other critical belongings. Within the building that caught fire, seven apartments burned and 23 more sustained smoke damage, Hicks said. Windows burst from their frames, leaving melted venetian blinds dangling free. The railing for the second-floor walkway was gone.

The temperatures overnight were below freezing. The parking lot was a sheet of ice after daybreak, with crews pouring down a powder mix to get traction.

It was believed to be the biggest fire in Everett since the November 2012 blaze at the downtown McCrossen Building, at 1814 Hewitt Ave. The Hodges fire the next year was a two-alarm fire, meaning fewer resources were deployed.

More than 90 firefighters from throughout Snohomish County had arrived on scene at the height of the New Year’s Eve blaze. They monitored hot spots overnight.

“We’re just making sure nothing flares up,” Hicks said.

The Bluffs apartments occupy 9.2 acres near the intersection of W. Casino Road and Evergreen Way, and include more than 200 units. The construction dates back to 1969, county property records show. That likely would predate local fire codes that require sprinklers in larger buildings.

The listed owner is a limited-liability company with a Hawaii address. The property, which sold in 2005 for $11 million, is part of the Hearthstone Housing Foundation, a low-income housing group with offices in Seattle and California.

Rikki King: 425-339-3449; rking@heraldnet.com.