Bronx, NY – A 77-year-old woman was killed and two other senior citizens injured in a fiery Bronx gas explosion that destroyed a three-story home Tuesday morning, police said.

The blast and subsequent raging blaze decimated the building on Fox St. near Intervale Ave. in Longwood, with an eyewitness recounting how the residence collapsed like a flaming house of cards after the 10:50 a.m. explosion.

“I just looked out the window as soon as I hear it,” said lifelong neighborhood resident Alex Catalan, 22, who lives a block away. “And I saw a bunch of smoke and fire coming from the house that’s collapsed … The pieces just went everywhere, the debris.”

The septuagenarian victim died at Lincoln Hospital, while an 82-year-old woman was critically injured and a 68-year-old woman was in stable condition at the Bronx medical facility, officials said. The older injured victim his since stabilized, police said.

Five police officers were also taken to Lincoln Hospital with smoke inhalation after working to rescue residents, authorities said.

“That whole building collapsed,” recalled Catalan. “I’ve never seen anything like it.”

The son of one victim said his mother and his aunt were inside the building at the time of the blast and he was now at a Bronx hospital awaiting word on their conditions.

“Imagine getting a call out of the blue to find out you could be losing your mom and auntie,” he texted.

Police sources and local elected officials said the explosion was apparently triggered by a gas explosion, and a Consolidated Edison spokesman said the investigation into the blast was ongoing.

“We are on location and we are working closely with the fire department to determine the root cause of the incident,” said Con Ed spokesman Alfonso Quiroz. “We shut off the gas to the block as a precaution.”

As firefighters began putting water on the fire, the building started to collapse, with the front facade falling onto a grey SUV parked outside.

Firefighters were ordered to back out of the building to douse the fire down from outside as more than 100 first responders, including firefighters, EMTs and paramedics responded to the scene.

A woman who lives across the street said everything happened in an instant: “A big boom and everything was moved all over. We came out the door and the house was on fire.”

The 60-something witness said neighbors took to the chilly streets with firehoses to keep the wind-whipped flames from spreading, although an adjoining building was damaged as well.

There were concerns that at least one person was possibly pinned inside when the explosion occurred, sources said. Initial searches of the rubble did not find any additional victims.

“Currently at the scene of a massive fire due to a gas explosion in my district,” wrote Councilman Rafael Salamanca Jr. on Twitter. “All emergency first responders are on the scene and currently battling the fire. I will continue to remain on site with my team as long as it takes to get answers and ensure the community’s safety.”

The blast comes just nine days after lethal smoke from a Bronx apartment building fire killed 17 people, the deadliest city fire in decades. That blaze was sparked by a faulty space heater and the smoke spread throughout the building after two self-closing doors failed to function properly.