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 neighborhood 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.