Four people were injured in an explosion at an apartment building in Torrance Sunday afternoon, officials said.

The incident was reported around 1:15 p.m. on the 23500 block of Anza Avenue, just north of the Pacific Coast Highway, Assistant Torrance Fire Chief Robert Millea said.

It appeared it was a gas explosion, but a hazardous materials team was on scene Sunday afternoon working to determine exactly what occurred. Anza Avenue had been closed for a few hours as emergency personnel responded.

The four victims were found suffering varying degrees on injuries.

Three of them were taken to a nearby hospital, authorities said, after earlier indicating all four had been transported. Officials were not aware of their exact conditions, but witnesses said some were seriously hurt.

Many in the neighborhood reported being startled by a large blast.

“They heard a loud explosion, kind of like a sonic boom,” said Jason Davis, who lives near the complex. “And then the people who were right by just felt a shake, like an earthquake.”

The explosion was mostly contained to two of the building’s units, with each sustaining significant damage. There was also minor damage to two adjoining apartments, according to Millea.

Workers were seen boarded up the windows of the affected apartments later Sunday afternoon.

The building has been undergoing renovations for some time, and the victims are believed to be remodelers working in one of the involved units, neighbors said. Some said they heard one of the workers may have turned on the gas inside one of the apartments before it was completely ready.

No further details were immediately available.