On Thursday, work crews remove debris from the smoking site of the explosion. (John Moore/Getty Images)

Rescue workers continue digging through the wreckage of the two buildings leveled in Manhattan, but a day after the explosion the only news was grim. They had found an additional four victims by Thursday morning, bringing the death toll to at least seven.

As more than 150 firefighters continued to work, removing debris and digging through the rubble in East Harlem, their work was complicated by the weather. Temperatures have plummeted, while gusting winds whipped up the fire still present at the wreckage.

Salvatore J. Cassano, the fire commissioner, has warned that it will “be a long, extended operation” to go through the debris.

“These rescue operations will continue for an open-ended period of time,” New York Mayor Bill de Blasio said during a press conference Thursday afternoon.

In addition to the seven confirmed deaths, about 40 people have been treated for injuries and taken to nearby hospitals, de Blasio said.

There are also still several people missing, though officials again declined to provide an exact number. Anyone trying to find information on a family member or a loved one who may have been in the area should call 311 from inside New York or 212-639-9675 from outside New York.

Numerous people in the area have also been displaced, with dozens of residences in surrounding buildings evacuated because the gas and electricity have been turned off. These buildings have been inspected and are structurally safe, de Blasio said.

A resident support center has been set up at a Salvation Army location nearby (175 E. 125th Street). City officials stressed that anyone impacted by the tragedy should seek help regardless of their immigration status.

An investigation is underway into the cause of the explosion, de Blasio said. In addition to city agencies, the National Transportation Safety Board has dispatched a team of investigators. Authorities need to get to the basement to look at the heating units and meters to see the possible source of the ignition or the leak, but they can’t do that until the fire is out and debris has been cleared, Cassano said.

The only warning before the explosion on Wednesday morning came in the form of a call to Con Edison about a gas odor in the area about 15 minutes before the blast, according to authorities.

John McAvoy, chief executive and president of Con Edison, said on Thursday afternoon that the utility had received two calls related to gas on the block over the last three years. One was in May 2013, the other was in January 2011. Both related to equipment inside buildings, and both were fixed, he said.

A preliminary review looking at 311 or 911 calls since 2010 didn’t find any related to gas odors in this area, William J. Bratton, the police commissioner.

But there has been a considerable uptick in gas-related calls to 311 since the explosion, Cassano said.

De Blasio, along with other officials, toured the area around the collapse on Thursday morning. He walked amid the smoldering debris, thanking a group of firefighters working on the site for their efforts. Watch de Blasio’s visit to the site here: