Long Beach Apartment Explosion Injures 19 : Accident: The blast on Atlantic Avenue flattens the storefront building and shatters windows nearby. Officials say the cause is not immediately known. One person is hurt critically.

At least 19 people were injured--one of them critically--when an explosion ripped through a storefront apartment building on a commercial thoroughfare in Long Beach on Monday evening, authorities said.

The blast in the 5500 block of Atlantic Avenue occurred at 5:59 p.m., flattening the four-unit building and shattering shop windows up and down the block, said Long Beach Fire Capt. Reed Bingham. Several nearby businesses, he said, were damaged and at least one car was blown off the street and onto a sidewalk where it plowed into a street light.

“It looks like a war zone,” Bingham said.

One man, a 35-year-old who firefighters believe had been living in the building, was critically injured and taken to Long Beach Memorial Medical Center with third-degree burns over most of his body, Bingham said.

Ten others, including two baby girls, were taken to local hospitals with lesser injuries, and at least eight more victims were treated at the scene, he said.

Bingham said the cause of the blast was not immediately apparent and is under investigation. A Long Beach police spokeswoman said authorities suspect it may have been a natural gas explosion. But Mike Milburn, who owns a plumbing business across the street, said he believes the gas had been shut off to the building some time ago, and added that immediately after the blast, he detected a heavy aroma of gunpowder.

Milburn, who was in his warehouse about 200 feet away when the explosion occurred, said the noise was deafening.

“My God, it was the worst explosion I ever heard,” he said. “It knocked me down.”

Milburn said it blew in the doors of his warehouse, briefly trapping him inside. Lumber, plaster and shards of roofing were still flying when he escaped, and he could hear people screaming up and down the block, he said.

The building, on a busy commercial strip, houses at least two upstairs residential apartments and two vacant spaces, one a former photographic lab and the other an abandoned video store, officials said.

Neighbors in the area said the building had been a favorite gathering place for squatters and transients, but Bingham said it was unclear whether the people inside the building were there illegally.

Times staff writer Shawn Hubler contributed to this report.