SAN JOSE –A string of downtown fires, including an early Saturday morning blaze that burned an elderly couple out of their Victorian home, is now thought to be the work of an increasingly brazen arsonist who is targeting San Jose as the city sleeps.

Ten fires have erupted across downtown San Jose since Wednesday: at a church, a warehouse, a commercial building and six homes. And fire officials announced Saturday that at least eight of the 10 fires were set intentionally, while two remained under investigation. All the fires occurred between the hours of 2 a.m. and 6 a.m.

Authorities released a police artist’s sketch of a “person of interest,” and the firefighters’ union announced a $10,000 reward aimed at capturing the man before he kills someone.

“These fires have targeted San Jose residents in the darkest hours of the night when they are most vulnerable,” said Robert Sapien, president of San Jose Fire Fighters IAFF Local 230, at a news conference Saturday. “We have been very fortunate that these fires have not resulted in serious injury or death to our residents. Should these fires continue, I fear this good fortune may run out.”

The latest blaze, which scorched a Victorian home on East St. John Street, began at 4:23 a.m. Saturday. The retired residents of the home, Sharon and Vincent La Vigna, were asleep when the fire broke out and were likely saved by their next-door neighbors.

“We saw the whole roof on fire. My dad and I were banging on the door, and no one woke up,” said Jose Lara, 25. “We broke a window and tore off a screen to get them out. My father and I were screaming ‘Come on, get out, your house is on fire!’ I grabbed the woman by the hand, and my father grabbed the man. They wanted to go back and get their cats. At first they didn’t understand how bad it was.”

San Jose Fire Station 8, on East Santa Clara Street, has responded to most of the blazes. At the solemn news conference held at Station 8 Saturday, fire officials urged the public to be on high alert. They said the police sketch was based on numerous tips, including video footage from home security cameras and an eyewitness who saw the man leaving the scene of one of the fires.

The man is believed to be a white or Hispanic man, age 25 to 40, who is at least 6 feet tall and weighs between 160 and 180 pounds. He has been spotted wearing baggy clothes and large frame glasses.

“We’re not sure what’s making this person tick,” said Capt. Chris Murphy, head the San Jose Fire Department’s arson unit. “It could be revenge, spite or mental illness.”

Murphy, who said he had slept just four hours in the last 48, noted that most of the blazes were remarkably similar in many ways.

“The fires have been in the same geographic area, they are similar in nature and they’ve occurred in the same time frame,” Murphy said. “In some cases papers or a newspaper was lit on fire and put through a mail slot. There’s no evidence of an accelerant, like gasoline, being used.”

The arsons began early Wednesday at Greater St. John Baptist Church on East San Antonio Street. On Thursday, there were five fires, including three at homes. On Friday, there were three fires. Saturday morning’s blaze was the 10th.

The arsonist tried to set one building on fire two nights in a row. The building at 1165 Peach Ct.is the home of a MGM Drywall. Early Thursday morning, someone stuck a burning object through the company’s mail slot, sparking other documents to catch fire. Early Friday morning, someone tried to pull a plywood panel off the side of the shop, stuffed newspapers inside, and lit the newspapers on fire.

Residents are urged to keep the exterior of their homes free of any debris that could quickly ignite — dry leaves or bundles of newspapers, for example — and are advised to make sure that all their smoke detectors are working.

Contact Dana Hull at 408-920-2706. Follow her at Twitter.com/danahull.