SILVER SPRING, Md. - First came an explosion that witnesses likened to a boom, a bomb or maybe even an earthquake. Then, a fire that torched a four-story apartment building, forcing people to drop their children from balconies and firefighters to rescue trapped residents from upper floors.

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 like Christmas decorations.

On Friday, authorities were still working to determine the cause of the blast that left two dead, dozens injured and others were 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 in the 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, including the three firefighters, were taken to hospitals for treatment. Hospital officials said most of those 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 neighbourhood just two miles from the District of Columbia border.

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

Erick Jimenez was desperately trying to get information about his father, Caesar, who he said lived on the first floor and was not answering calls.

"I've gone to the emergency rooms, Holy Cross, Shady Grove, nobody knows," Jimenez said.

Eddie Macario said he lives in the building where the fire started.

"We were sleeping when we felt everything started shaking. We thought it was an earthquake, or just a tremor. And when we came out to the living room, it was full of smoke. We went out to the hallway and all the neighbours were getting out, running. A lady even said she had lost her baby in an apartment, I went in to see if I could get him, but I couldn't; it was collapsing, full of smoke. Nothing could be done, and she lost her baby there, and up until now they haven't found him."

Mariama Turay, who has lived in the apartment complex for 30 years and spent the night in the shelter, said she chooses to view the explosion as a blessing, so loud that it woke everyone up and alerted them to get out.

"It was just like a bomb fell," she said. When she escaped her first-floor apartment, the fire was "right in my face."

Firefighters found people on upper floors who needed help, Montgomery County Fire Chief Scott Goldstein said at a briefing. They used ladders to rescue them.

"People were dropping children and jumping out of other windows," Goldstein said. "Everybody was getting out of the building as rapidly as possible."

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 neighbourhoods 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 established at a nearby community centre.

Carden said those displaced are primarily Spanish speakers. Translators were being brought in to ease communication. Officials made comments in English and Spanish during a Thursday news conference.

Carden said the nature of the explosion was especially traumatic.

"To basically be in an event where people were blown out of their beds ... and what they own is lying across the street, that's a key difference" from a typical fire evacuation," he said.

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Associated Press writer Sarah Brumfield in Washington and videographer Bill Gorman in Silver Spring, Maryland, contributed to this report.