Calling it a "devastating loss to the state of Texas," Maldonado did not provide details on how the fire may have been set or whether authorities had identified a suspect.
"There were some working cameras and surveillance video and all of that is useful to us and that is part of what we are using to help us with this investigation," Maldonado said.
"There was security personnel on the scene but we do have some evidence that indicates that there was somebody here that intentionally set the fire."
Fire officials said they hadn't had a chance to thoroughly investigate inside the mansion.
Investigators also were interviewing people who were nearby and might have noticed suspicious activity at the mansion, which sits downtown on a lot the size of a city block.
"We're going to come get the person that's responsible for causing this damage," Maldonado said.
About 100 firefighters responded when an alarm went off just before 2 a.m. The mansion has been unoccupied since Gov. Rick Perry and his wife, Anita, temporarily moved out last fall so the mansion could undergo a $10 million renovation. It was scheduled to be completed next year.
No injuries were reported, and there was no immediate financial estimate of the damage.
"We are heartbroken by the fire that has ravaged the Texas Governors Mansion," Perry, who was out of the country, said in a prepared statement. "It has not only been our home for the past eight years, but has stood as a symbol of Texas pride throughout its history. Though it can certainly be rebuilt, what Texas has lost today can never be replaced."
Small hot spots were still smoldering more than six hours after the fire began. Puffs of smoke billowed from the building.
State officials said damage to the Greek Revival-style mansion — first occupied by Gov. Elisha Marshall Pease — was extensive.
The roof buckled because of the fire and the massive amount of water used to extinguish it. The mansion was left black with heavy burns. Parts of the six 29-foot columns at the front of the home and much of the front wall of the mansion were charred black. In some places the original color of the brick could be seen where white paint had burned off.
Perry spokesman Robert Black said much of the wood inside the mansion was longleaf pine that is "completely irreplaceable." He said some interior ornamentation is damaged beyond repair.
All the historic furnishings and heirlooms had been removed from the mansion for the renovation project. Braun & Butler Construction was working on the maintenance and renovations. Among the improvement projects were an overhaul of the plumbing, removal of lead paint and asbestos and installation of a fire sprinkler system, among other things.
The mansion was equipped with a fire alarm. A state trooper who was on the mansion's grounds as part of regular security detail heard an alarm go off, then 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 erupted. They said there is no evidence any direct threat to the governor was intended.
The governor uses the mansion as a home and for official functions, such as hosting heads of state or other dignitaries and for gathering with lawmakers and the news media.
The Perrys have been living in a rented home elsewhere in Austin during the renovation work. They were in Stockholm, Sweden, on Sunday as part of a European economic development trip and were scheduled to return to Texas on Tuesday. The governor was contacted shortly after the fire was reported, and his chief of staff was keeping him updated on the situation, Black said.
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.
The large trees surrounding the two-story mansion made it difficult for firefighters to position their equipment, Fire Department spokeswoman Dawn Clopton said.
The federal Bureau of Alcohol, Tobacco and Firearms was sending in a national response team to assist the investigation. Maldonado asked that anyone who has information about the fire to call a hotline at 1-877-434-7345.