(CNN) -- Some thought it was a bomb. Others thought it was the commuter train that runs behind their buildings jumping the tracks.
It shook upper Manhattan for blocks -- and when it was over, a five-story apartment building and its neighbor were gone.
After Wednesday morning's deadly building explosion in East Harlem, squads of firefighters dug through piles of shattered bricks and beams. Ladder trucks sprayed water into the gap where the buildings once stood. As Detective Martin Speechly, a New York police spokesman, put it: "1644 Park Avenue appears not to be there anymore."
Along with that five-story apartment building, with a Latino evangelical church on the first floor, a neighboring piano store and the four floors above it collapsed in the blast.
One nearby resident, Angelica Avila, told CNN she was trapped in her apartment down the block for a short time afterward. Her stepmother had to sneak in through the back to try and open the door for her, she said.
"My neighbors came banging on my door. I guess they were evacuating the building, and I couldn't get out -- my door was jammed," she said. "Everything off my windowsill fell, and I guess the impact of the explosion jammed the door as well."
Something similar happened to Aisha Watts, who had just returned to her apartment in one of the adjoining buildings after taking her children to school. Then the windows broke out, and "the walls came tumbling down," Watts said.
A neighbor helped her out of her apartment, because the door was stuck in its frame. And she and her neighbors will have to find someplace else to stay temporarily.
"We can't go back for the next couple of days," Watts said.
Three blocks away, the blast knocked Klay Williams off his feet as he brushed his teeth before work. He told CNN's sister network HLN that his first thoughts were a possible terrorist attack, "as New Yorkers, we tend to do," or maybe a derailment of the nearby Metro-North commuter line.
"I went to my back window, because I'm on the very top floor," he said. Looking down the street, he saw "a bunch of people just running, as if they were running towards something."
Authorities suspect the explosion was the result of a gas leak reported a short time before.
Seven blocks away, Eric Boise could feel his apartment shake and saw others "pouring out of the apartment buildings at 116th and Madison Avenue." He followed himself, watching as firefighters hauled gurneys into the wall of smoke to bring out the wounded.
"I can see in front of me about 50 feet up until the explosion, and then it gets pretty thick," Boise told CNN.
Michael Mowatt-Wynn, the head of the Harlem & the Heights Historical Society, called the surrounding area "a community in transition." Its population of 100,000-plus was once largely Italian. In the late 1950s, large numbers of Puerto Ricans moved in -- in part because of the similarities between their native Spanish and the Italian still spoken there.
The 1990s brought a growing Mexican population, and now the area is being gentrified, Mowatt-Wynn said. It's home to a lot of mom-and-pop businesses and restaurants, and the City University of New York-Hunter College is building a new dormitory in the area, he said.
The full toll was uncertain early Wednesday afternoon, but at least three people had died, dozens were hurt and about 10 were unaccounted for. The suspected cause was a gas leak that had been reported only 15 minutes before the explosion, Mayor Bill de Blasio told reporters.
"This is a tragedy of the worst kind, because there was no indication in time to save people," he said.
Among the missing was 67-year-old Carmen Tanco, whose nieces and godson awaited word near the ruins of her building. Liz Robinson said the family had given her aunt's name to the office of her city councilwoman, which was canvassing hospitals to track down the injured -- "and she's still not on any of those lists."
"She's a dedicated worker. She works at a doctor's office for many years now. So they're worried as well," Robinson said. "They haven't heard from her. Today's her assigned day off."
Another niece, Marisela Frias, said her "sassy, spicy" aunt rarely fails to answer her phone -- "and if she misses it because she didn't get to it in time, she calls me right back."
"I'm hoping, thinking that the reason she hasn't been in one of the lists is because she's so involved helping somebody else that was injured from the building -- that she hasn't taken toll to herself because that's how she is," Frias said.
CNN's Poppy Harlow, Rose Arce, Chris Kokenes and Ray Sanchez contributed to this report.