The 9:30 a.m. blast blew out windows, lifted nearby residents off their feet and sparked a fire that sent up plumes of smoke visible 10 miles away.
Several residents who fled the 24-unit building said they heard an elderly woman screaming for help because her husband was trapped inside the apartment closest to the blast. It was not immediately clear whether the man was one of the three people killed; the bodies were still inside the three-story building late Tuesday afternoon.
"I wanted to go in and look for him but there was no way," said David Coradin, 54, who was taking a shower in his apartment when the explosion blew out his front door and shattered his windows.
Most of the building's roof and about a third of the second floor had collapsed, authorities said.
Police Chief Thomas Lucas said four residents had not been accounted for but may have left the building before the blast.
Officials said preliminary investigations showed that a pit dug by construction workers who were trying to remove an underground oil tank collapsed and pinched a gas line just before 9 a.m.
Utility crews were sent to the scene after residents nearby reported the odor of gas. Thirty 30 minutes later, the explosion shook the area.
Bergenfield Fire Chief John Pampaloni was on the scene because of the gas leak when the explosion occurred. He said he had not ordered the building evacuated because he didn't believe it was a dangerous situation.
The five injured were taken to hospitals. One of them, a woman, was reported in critical condition with second- and third-degree burns.
Richy Harps was in his apartment in the building next door when the blast hit.
"I was listening to music at the time and I heard an extremely loud explosion, which literally levitated me off the ground," he said. "Everyone was running outside clutching their clothes. It's a horror."
Mayor Richard Bohan said about 12 families were left homeless.
Bergenfield, with a population of 26,000, is located six miles northwest of New York City.