SAN JOSE -- Possible human remains were detected Wednesday in the rubble of a five-alarm fire that destroyed the former KNTV building over the weekend, but officials will have to bring in an excavation team Thursday to confirm that.

Also Wednesday, a homeless man was charged with using a liquid accelerant to burn down the building whose last incarnation was a haven for the city's homeless.

Stillman Pfeffer, 55, of San Jose, was charged with arson and first-degree burglary for the fire on Park Avenue, according to the Santa Clara County District Attorney's Office. and arson investigators with the San Jose Fire Department.

Pfeffer was arraigned Wednesday and did not enter a plea. He is being held without bail while dogs trained to detect human remains are deployed by the San Jose Fire Department to examine the charred structure.

About 1:30 Wednesday, one of the dogs alerted handlers of a possible hit in the rubble. A short time later a second dog had a hit in the same location. The pile of debris was then partially excavated and the dogs were brought back again in the early evening. Again, they detected something, but officials said they would have to do a deeper excavation Thursday morning.

San Jose Fire Capt. Cleo Doss said that while they couldn't confirm remains were under the debris, the dogs used are generally quite reliable.

"These dogs are very experienced," he said. "One just came back from the state of Washington where it was assisting in the mudslides. They've done a lot of work and been very reliable. The handlers are very professional. But we still have to confirm what they say we have."

He said if remains are found, police will take over the crime scene.

"If there's something there, we will find it," Doss said.

If remains are found, Pfeffer would face more serious charges. If they aren't, his bail will be reassessed, Deputy District Attorney David Boyd said. A witness to the fire told investigators she saw a man "squirt a liquid onto a mattress" seconds before the fire broke out, according to an SJFD arson report. A dog specially trained in detecting fire accelerants helped corroborate the claim.

The witness also said she had a 4-month-old puppy that died in the fire. She helped direct San Jose police and the fire department to Pfeffer's location not far from the fire scene.

More than 100 firefighters from San Jose, Santa Clara and Sunnyvale battled the fire, which broke out at 3 p.m. Sunday. It was brought under control about two hours later but not before the roof and sides of the building collapsed. Two firefighters suffered minor injuries. Damage was estimated at $4 million.

The building was once home to KNTV but had been vacant since 2004. In 2012, it was used for fire training and recently as many has 30 homeless people have stayed overnight there. Pfeffer was reportedly homeless himself but was not staying in that building. Pfeffer is on probation for petty theft and had three prior convictions, authorities said.

The building was purchased by the city of San Jose's Redevelopment Agency in 2008 during its efforts to buy land in the downtown area for a prospective baseball stadium in an attempt to bring the Oakland Athletics to the city.