Update at 8:15 a.m .ET, March 13:
We've begun a new post, which at this moment is headlined "Death Toll From NYC Explosion Stands At Seven, May Go Higher."
Update at 4:25 a.m. ET, March 13:
A sixth person has been confirmed dead in the explosion that flattened two apartment buildings in Harlem. A fire department spokesman says the unidentified body was pulled out of the rubble around 3:15 a.m.
Update at 3:55 a.m. ET, March 13:
The explosion that destroyed two apartment buildings in Harlem on Wednesday has claimed the life of a fifth person. The body of an adult female was found around 2:50 a.m. Thursday, according to The Associated Press. At least five people are unaccounted for.
Update at 2 a.m. ET, March 13:
A fourth person has been confirmed dead in Wednesday's explosion, according to fire department spokesman Jim Long. He says the unidentified adult male was pulled from the rubble shortly after midnight. Nine people are missing and more than 60 are injured.
Our original post and an update from 6:55 p.m. ET:
New York firefighters were searching two East Harlem buildings for possible survivors after an explosion and collapse of the structures, including a five-story apartment complex. Officials say that a preliminary investigation indicates a gas leak as the likely cause. At least three people are dead, 36 injured and several missing.
New York Mayor Bill de Blasio's office said Wednesday evening that the death toll had gone up to three, from the two dead reported earlier. The mayor's office said nine were missing.
The explosion occurred at 1646 Park Ave.
De Blasio, speaking at news conference earlier in the day, described the incident, which occurred at about 9:30 a.m. ET, as "a tragedy of the worst kind."
He said that there were people still missing from the blast that affected buildings in an around the center of the explosion. He said the response to the explosion would "be an extended operation."
Fire Commissioner Salvatore Cassano said the building where the explosion was thought to have occurred "appears not to be there anymore."
Con Ed tweets that its workers were responding to a report of a gas odor at 1652 Park. The utility says the call came in at 9:13 a.m. and that crews were dispatched a few minutes later and arrived on the scene after the explosion.
De Blasio said preliminary reports are that there was no indication of a problem before that call.
It's the first major emergency since de Blasio took office.
Officials say there was no major damage to other buildings in the area, but Beverly MacFarlane, who lives a block away, tells NPR's Joel Rose that when she rushed back from work after hearing about the explosion, she found "my pictures ... on my wall [were] on the floor."
Live television footage from the scene shows at least four fire department ladders squads pouring water on the burning debris as heavy smoke billowed off the wreckage.
The New York Times says witnesses reported hearing what sounded like an explosion before the collapse occurred.
CBS New York quotes one witness, Samuel Paul, as saying the building suddenly shook.
"We saw a whole lot of smoke. A lot of smoke came out," Paul told CBS 2. "There's a lot of dark smoke still coming out. A lot of fire engines I saw going to 125th Street."
"The smoke started to rise. It looked like something fell because it wasn't like a fire. It just looked like debris smoke, similar to 9/11," he added.
The Daily News quotes another witness, Ashley Rivera, as saying "for weeks [tenants] have been smelling gas."
Rivera said when the explosion occurred, "We saw people flying out of the window ... Those are my neighbors."
Reuters says commuter trains were stopped on nearby tracks and passengers were ordered off the Metro-North Railroad cars at the Fordham stop in the Bronx, passengers said.