New York (CNN) -- One minute, Colin Patterson was watching TV. The next, he saw pianos flying through the air in the shop where he works as an explosion tore through the building.
"They flew off the ground," said the piano technician, who also lives in the building in Manhattan's East Harlem. He told CNN affiliate WABC that he crawled through the rubble and managed to escape unharmed.
At least four people were killed in the massive explosion and fire Wednesday that leveled Patterson's building and the one beside it.
Nine people remained missing hours after the blast, city officials said. Firefighters were still frantically picking through rubble in search of survivors.
Authorities say they think a gas leak was to blame, but they haven't determined an official cause yet.
The massive explosion and fire leveled two five-story apartment buildings. At least 63 people were reported injured.
More fatalities appeared likely. Fire officials reported that two survivors suffered life-threatening injuries.
Near 116th Street and Park Avenue, once the heart of New York's large Puerto Rican community, about a dozen firefighters tore at two-story-high mounds of bricks in a search for survivors from the two buildings, which housed a piano store and an evangelical church in addition to apartments.
As gas and electric utility workers tore up pavement in an effort to shut gas lines, people gathered in the streets, many crying.
"This is a tragedy of the worst kind," New York Mayor Bill de Blasio said, "because there was no indication in time to save people."
Call about gas leak proceeded blast
Con Ed spokesman Bob McGee said the utility received a call reporting a gas leak around 9:13 a.m. The call came from a resident at one of the newer buildings on Park Avenue. The utility dispatched a truck two minutes later, but it arrived after the explosion, the spokesman said.
Edward Foppiano, the utility's senior vice president of gas operations, said the cause of the blast was being treated as a gas leak, though there was no evidence of that yet. A routine service check, involving a truck with special equipment, detected no gas leaks on February 28, he said.
Foppiano said one of the buildings had a gas leak in May 2013, which was repaired the following month.
A building department official said one of the two Park Avenue buildings that collapsed received a city permit last year for the installation of 120 feet of gas piping. The work was completed last June. In 2008, owners of the adjacent building, which also collapsed, were fined for failing to maintain vertical cracks in the rear of the building. The condition was not reported as corrected to the buildings department.
There were a total of 15 units in the two buildings, officials said.
Building department records detailed a litany of violations, dating back decades, for one of the collapsed buildings, including a lack of smoke detectors, blocked fire escapes and faulty light fixtures.
The National Transportation Safety Board, which investigates pipeline explosions, said it was sending investigators to New York.
The mayor told reporters that the report of the gas leak, which he said came about 15 minutes before the explosion, was "the only indication of danger."
Longtime resident, public safety officer among victims
Marisela Frias, 44, tried unsuccessfully to call her 67-year-old aunt, Carmen Tanco, a second-floor resident of one of the collapsed buildings for more than two decades. Frias called and called her aunt's cell number but got no answer, she said.
"Always she answers right away and if she misses it because she didn't get to in time, she calls me right back," Frias said.
"Calling nonstop and it goes straight to voice mail, which never ever, ever, ever in my years -- and I'm 44 -- of knowing her, ever cannot get ahold of her. ... All I want to know is she's OK."
Later Wednesday, Frias told CNN that her aunt had died in the blast.
Another one of the victims was a public safety officer at Hunter College, the school said on its website.
"We are sad to report that, in an explosion that destroyed two buildings in East Harlem this morning, we have lost a member of the Hunter family. Sgt. Griselde Camacho, a public safety officer at Hunter since 2008, who worked in the Silberman School of Social Work building, died in the explosion," the statement said. "Our hearts go out to Griselde's family at this terrible time."
The name of the other person who died in the blast was not immediately released.
The injured included two FBI agents who were in the vicinity of the explosion, said New York FBI spokesman Chris Sinos. The injuries were not life-threatening.
One woman tried in vain to find her husband, Jordy Salas, who may have been on the second floor of one of the collapsed buildings. She fainted and was taken to a hospital.
"We're expending every effort to locate each and every loved one," de Blasio told reporters at the scene. "Hopefully we'll find that some of them are in other parts of the city and have just not been located yet."
Neighbors: Blast sounded like a bomb
Fire Commissioner Salvatore Cassano said responding firefighters barely missed the blast.
"If we were here five minutes earlier we may have had some fatalities among firefighters," he said. "Not being here may have saved some lives."
A new problem complicated search efforts Wednesday night, as a sinkhole in front of the collapsed buildings stopped firefighters from accessing some of the wreckage.
"Heavy equipment, required to remove additional debris, cannot be brought to the scene unit the sinkhole is mitigated," the New York Fire Department said. "That mitigation is in process and will likely take several hours."
Clouds of dark smoke rose over the largely residential area of redbrick tenements and small businesses after the explosion, which some residents said sounded like a bomb.
Hundreds of firefighters responded, many spraying water on the roaring blaze from ladders.
Metro North commuter rail service was suspended as debris from the explosion landed on the elevated tracks across the street, authorities said. Service was restored later Wednesday, with trains running at reduced speed near the blast site.
"I heard a big explosion," said a resident who identified herself as Angelica. "I didn't know what was going on. ... My neighbors came banging on my door, telling me to get out. I guess they were evacuating the building. And I couldn't get out. My door was jammed. Everything on my windowsill fell. I guess the impact of the explosion jammed the door as well."
She added, "It was extremely loud. I couldn't even explain it to you, if I could. It was just so loud. It woke me out of my sleep. That's how loud it was."
Molley Mills, who lives nearby, said at the time of the explosion her building rumbled as if the subway was passing beneath it.
"I went outside my terrace and there was smoke pouring out," she said.
The New York police bomb squad responded to the scene, according to a law enforcement source.
Once a predominantly Italian neighborhood, the stretch of East Harlem saw a large influx of Puerto Ricans in the 1950s. It went on to be called Spanish Harlem. In the 1990s, many Mexican immigrants began to move into the area, which has been gentrified in recent years, with many mom-and-pop shops replaced by restaurants and bars.
CNN first learned of this story via Twitter.
CNN's Poppy Harlow, Rose Arce, Eden Pontz, Don Lemon, John Berman, Ashleigh Banfield, Adam Reiss, Stephany Byrne, Haley Draznin, Laura Ly, Shimon Prokupecz, Brian Vitagliano, Julia Lull, Lorenzo Ferrigno, Steve Kastenbaum, Elizabeth Landers, Susan Candiotti, Haimy Assefa, Chris Boyette, Kevin Conlon and Catherine E. Shoichet contributed to this report.