No one was inside the 152-year-old mansion at the time, said Robert Black, a spokesman for Gov. Rick Perry. The mansion had been undergoing a $10 million renovation, and Perry and his wife, Anita, had moved out last fall.
"We have some evidence that indicates that we do have an intentionally set fire," said state Fire Marshal Paul Maldonado. "So we believe that we may be looking at a criminal act here."
The federal Bureau of Alcohol, Tobacco and Firearms was sending in a team to help investigate, Maldonado said. He did not give details on how the fire may have been set or whether there was a suspect, but said there were security cameras on the premises.
State officials said they hoped the Greek Revival-style mansion had not been destroyed, but the damage clearly was extensive.
"It is bordering on catastrophic. The roof has not yet collapsed. The structure is still intact," said Allison Castle, a Perry spokeswoman.
However, the roof had buckled in places and the building was blackened, including parts of the six 29-foot columns standing at the front. In some places, white paint had burned away to reveal the original color of the brick.
"They built them pretty sturdy back then," Black said. "Nonetheless, it's taken quite a beating this morning."
Black said some interior ornamentation was damaged beyond repair. Two white front porch swings appeared to be unscathed. There was no immediate financial estimate of the damage.
The mansion is a national historic landmark. Built in 1856, it is the oldest continually used executive residence west of the Mississippi, according to the group Friends of the Governor's Mansion, which works to preserve and show the public the historic building.
About 100 firefighters were sent to the four-alarm blaze, Fire Department spokeswoman Dawn Clopton said. Small hot spots were still being extinguished more than five hours later.
A state trooper who was on the grounds as part of regular security detail heard the mansion's fire alarm, saw flames and called the fire department, said Tela Mange, spokeswoman for the Texas Department of Public Safety.
Officials would not comment on how many security officers were present when the fire started. They said there is no evidence any direct threat to the governor was intended.
Perry and his wife were in Stockholm, Sweden, on Sunday as part of a European trip. They are scheduled to return Tuesday.
The governor was told about the fire, Black said. "His first concern was if anyone got hurt," Black said, adding that Perry also expressed sadness over "the loss of a Texas treasure."
The mansion had a fire alarm but no sprinkler system. Black said installation of a fire suppression system was among the renovations that began in October.
The large trees surrounding the mansion, which sits on a lot the size of a city block, made it difficult for firefighters to position their equipment, Clopton said.
The governor uses the mansion as a home and for official functions, such as hosting heads of state or dignitaries.