SAN JOSE -- Firefighters Friday afternoon were still tending to a deadly four-alarm blaze at a San Jose warehouse to monitor for flare-ups as an investigation into what sparked it continues, San Jose Fire Department Capt. Craig Schwinge said.

"We're going to be here all night, as a fire watch," said Schwinge.

Shortly after the fire was reported at 4:55 a.m. inside a commercial building on Faulstich Court near Oakland Road, emergency crews found a man dead inside, according to Schwinge.

The man has not been identified by the Santa Clara County medical examiner's office, and investigators are not sure what he was doing there.

Firefighters called a fourth-alarm at about 5:30 a.m. and were prepared to raise it to a fifth as daylight approached but it was not needed.

Schwinge said firefighters were able to knock down the bulk of the fire by mid-morning, but smoke was still billowing this afternoon from the corrugated metal structure.

Fire Capt. Kraig Schwinge told KTVU at the scene that one of the major challenges presented by the fire was that firefighters did not know what was stored in the building.

Though the building's steel frame construction is still mostly intact -- inside, boxes, wooden pallets and a black rubbery material Schwinge said is used with Astroturf went up in flames.

Schwinge had no estimate of the damage, and said an engineer will have to evaluate whether it needs to be torn down.

The warehouse was being used by at least two businesses, he said.

Fire investigators and investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives are probing the cause of the conflagration.

Schwinge declined to say whether he considered it suspicious. "It's under investigation," he said.