NEW YORK (CNN) -- A gas leak unleashed a massive explosion and raging fire in Manhattan's East Harlem Wednesday, leveling two five-story apartment buildings, killing two people and wounding dozens of others, authorities said.
A law enforcement source said that about 10 people remained missing hours after the blast. Firefighters were still frantically picking through rubble in search of survivors.
The number of injured climbed through the afternoon: At least 52 were reported injured. Harlem Hospital received 13 patients, including a child in critical condition, a hospital spokeswoman said.
Mount Sinai Hospital was treating 22 patients, including a woman with head trauma in intensive care. Three were in serious condition, a spokeswoman said earlier Wednesday. Many patients came in on their own. Some of the patients were children.
Metropolitan Hospital Center received 17 patients; nine adults and one child remained under evaluation in stable condition.
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.
The identifies of the two women who died were not available.
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 -- a piano store and an evangelical church.
As gas and electric utility workers tore up pavement in an effort to shut gas lines, people gathered in the streets, many crying.
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," Mayor Bill 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."
The cause was unclear, but Con Ed spokesman Bob McGee said the utility received a call about a gas leak at 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.
"This is a tragedy of the worst kind, because there was no indication in time to save people," de Blasio said.
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 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."
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."
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.
"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.