CONCORD (CBS SF) — Even as demolition crews were set to begin the challenging task of removing tons of charred debris from the site of a Concord apartment complex construction site fire Thursday, lingering flames could be seen inside the smoldering remains of the massive building.

Officials said the cleanup would begin early Thursday of the debris that fell into a neighboring street when a towering 5-story scaffolding structure attached to the side of the building collapsed.

The goal, fire officials said, is to allow more than 200 residents of a neighboring apartment complex to return to their homes for the first time since the fire erupted early Tuesday.

Concord Fire Marshal Robert Marshall said firefighters hoped to be able to gain access to the area adjunct to the evacuated apartments and “get inside to knock down the fire.”

The “stubborn hot spots” were also producing a smoky haze in the neighborhood.

Arson investigators from the Concord fire department, police department and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives have yet to enter the ruins to attempt to determine if the 3-alarm blaze was intentionally set or the result of an accident.

The fire was reported at 12:55 a.m. Tuesday at an apartment building under construction near the intersection of Clayton Road and Galindo Street.

Two people were taken to the hospital with smoke inhalation and more than 200 people were evacuated from a neighboring apartment complex. Firefighters have given a primarily estimate of $55 million for the damage caused by the blaze.

County health officials warned area residents to avoid direct contact with and inhaling ash and debris from the fire. Those materials can be cleaned with soap and water and the ash should be dampened before sweeping to minimize the generation of dust.

The pollution from the fire could accumulate in nearby creeks and rivers, so people who live nearby should avoid sweeping any ash or debris into storm drains where it might adversely affect the watershed.

Marshall said the building was the second phase of the Renaissance Square apartments. It was 60 percent complete and a mostly wood structure.

The sight of ATF arson investigators at a fire scene have become relatively common in the Bay Area. They were called in to assist in the search for the cause of Oakland’s deadly Ghost Ship Warehouse fire in 2016.

They also have been part of the investigations of three large fires at buildings under construction in Oakland and Emeryville in 2016 and 2017.

In July, ATF officials released photos of a man they said was an arson suspect and announced a $100,000 reward for information leading to his arrest and conviction.