AUSTIN – Six months after an arsonist torched the Texas Governor's Mansion, the blaze remains unsolved. And some arson experts say it may stay that way.

The Department of Public Safety remains confident that investigators will find the perpetrator, but with an apparently fruitless surveillance video, an unclaimed $50,000 reward, and one lead investigator with other cases on his plate, seasoned fire investigators say the only hope now is for someone to spill the beans.

"Unless you've got a written confession [or] a picture of someone recognizable with a match in their hand, you're often out of luck," said Terry Aikman, president of the Central Texas Fire Investigators Association. "I hate to say it, but sometimes the evidence just isn't there."

DPS spokeswoman Tela Mange said agency investigators continue to follow leads and are doing everything they can to crack the case. She said that while arsons can be difficult to solve, "it just takes one piece of solid information to break a case wide open."

"We urge anyone who might have information that may be helpful to contact us," she said.

The Governor's Mansion went up in flames before sunrise on June 8, lit by a young man captured on grainy surveillance footage hurling a Molotov cocktail at the 152-year-old building's front door. No one was injured in the fire, which took several hours to put out.

Internal DPS investigators later found that their troopers violated at least six policies and procedures that contributed to a poor response to the blaze and that the only officer on duty wasn't patrolling the grounds as he was supposed to. No DPS employees were punished.

It's still unclear how much it will cost to restore the residence, which was vacant for renovations at the time of the blaze. Nor can investigators put a timeline on solving the crime.

Arsons are notoriously hard to solve, said Dr. Gerald Hurst, a chemist and Austin-based fire forensics expert. Evidence – in this case probably a glass bottle and a gasoline-drenched rag – is often destroyed, and rarely carries fingerprints. Authorities have declined to discuss what physical evidence, if any, they have recovered from the fire.

Without a clear motive – someone burning down their own home to get an insurance settlement, for example – it's very difficult to pin down a suspect, Dr. Hurst said.

"I'm sure they've done all the facial recognition [on the surveillance footage] they can," said Mr. Aikman, who is the deputy fire marshal in Schertz, a San Antonio suburb. "I know they're working very hard at it. But without an eyewitness, sometimes it just doesn't come to an arrest."

Nationally, just 15 to 20 percent of arsons result in an arrest. Mr. Aikman said that in his experience, fewer than 5 percent of these intentional fires end with convictions. But Dr. Hurst said he thinks it's likely this fire will be solved someday – because "eventually, someone shoots their mouth off."

"What you've really got is a whodunit. A 'who the heck was he?' " Dr. Hurst said. "If they solve this case, it will be because somebody brags about it."

So far, no such bragging has been reported. Two men accused of bringing Molotov cocktails to this summer's Republican National Convention in Minnesota were evaluated as possible suspects in the mansion fire, but there doesn't appear to be evidence linking them to the Texas case.

An attorney for one of the young men, who are being held in Minnesota, said his client was never approached by Texas officials.

The lead DPS investigator, 42-year-old Sgt. Jimmy Schroeder, declined to comment on the case. Sgt. Schroeder, a 15-year veteran of the agency, was assigned to the case because he oversees Travis County, DPS officials said.

"We're certainly all concerned that we to date have not apprehended the person or persons responsible for the fire," said Allan Polunsky, chairman of Texas' Public Safety Commission, which oversees DPS. "I'm satisfied that [DPS] is focused and committing adequate resources on this investigation."

A $50,000 reward is available for information leading to an arrest in the fire at the governor's mansion. Call 1-800-252-TIPS.