People look over the damage and debris field Thursday left from the explosion and fire at an apartment building in Silver Spring, Md. Residents reported smelling gas prior to the blast.

SILVER SPRING, Md. — First came an explosion that witnesses likened to a boom, a bomb or maybe even an earthquake. That was followed by the fire that torched a four-story apartment building.

Finally, the aftermath: a debris field scattered over 100 yards, including a door blown across a two-lane road and parking lot, and clothes hanging from trees.

Authorities today were still working to determine the cause of the blast that left two dead, dozens injured and others unaccounted for. The Bureau of Alcohol, Tobacco and Firearms was assisting the investigation.

Among those injured were three firefighters who were treated and released Thursday. Officials described heroic efforts to save people trapped by fire.

The two found dead in the debris have not yet been identified. Residents reported smelling gas prior to the explosion, and officials responded to calls for a potential gas leak at the complex last month.

In all, 34 people were taken to hospitals for treatment. Officials said most were released later in the day.

People more than a mile away reported feeling their homes shake in the blast just before midnight Wednesday night in a working-class neighborhood just two miles from the District of Columbia border.

Nine hours later, firefighters were still using a hose to put out the last of the fire and smoke was still rising from the building.

Initially, fire officials said they were looking for five to seven people who were unaccounted for. Police later said they don’t have a firm number. Officials said they suspended rescue efforts in the debris and switched to recovery mode. The building was still at risk of collapsing, so it had to be shored up before recovery efforts and the investigation could continue, County Fire and Rescue Service Division Chief David Steckel said.

The Red Cross was working with families to determine their needs. While Montgomery County is one of the nation’s wealthiest communities, the neighborhoods inside the Capital Beltway in this section of Silver Spring are working-class areas, home to large numbers of Central American immigrants.

Paul Carden, regional disaster director for the American Red Cross, estimated that 100 people were displaced and 60 or 70 were staying at a Red Cross shelter nearby.