Although two electrical appliances were in the attic, it was determined neither could have sparked the fire. Instead, Wilson and White said evidence at the scene pointed toward arson.
White said a vent door leading to the attic was open and placed "neatly" against a wall.
He added beneath the attic was a storage facility where there were signs of drug use, with both spent and unlit matches scattered.
White said when fire crews arrived at the scene Wednesday night the door to the storage room to the right of the front entrance was found open.
"Something was thrown into the attic from the storage room," White said, adding the fire will continue to be under investigation and the search will begin for suspects.
The blaze started around 9:52 p.m. Wednesday night, with a second alarm being called thereafter.
El Centro fire Capt. Bill DuBois said for the first 20 minutes of the blaze on- and off-duty El Centro crews attempted to battle the flames inside the theater.
"They did a real aggressive early attack with hoses inside," DuBois said, but added after 20 minutes crews were ordered out of the building.
"After 20 minutes you start to worry about collapse," DuBois added.
The fire reached three-alarm status and fire crews from Brawley, Holtville, Imperial County and Calexico were called in to assist, El Centro fire Capt. Steve Seibert said this morning.
DuBois said all three ladder trucks in the Imperial Valley — from El Centro, Brawley and Calexico — were used to extinguish the fire.
Seibert said the roofs of the theater as well as the other two buildings collapsed. He added sections of walls facing both Seventh and State collapsed into the roadway.
At one point, Seibert said, containment and not allowing the fire to spread to other structures downtown were key concerns.
No firefighters were injured battling the blaze. Seibert said there was no indication anyone else was hurt.
Efforts to reach Fox Theatre owner Chris Van Snellenburg of Vancouver, British Columbia, Canada, were unsuccessful this morning.
However, Jesse Nuñez, who runs the Tigre Negro Kung-Fu studio, said everything he had built over the past year and half was destroyed by the fire.
"All the investment we did, we lost everything," Nuñez said as he surveyed the loss to his business.
Nuñez said he was called to the scene at 10 p.m. Wednesday and remained there until 2 a.m. He said his business suffered about $20,000 in damages.
Still, Nuñez said he is not giving up.
"We will try to start again and get another building," he said.
A damage estimate was not available this morning.
In early December, The Salvation Army Thrift Store in the 600 block of Main Street was destroyed by fire. A cause of that blaze has never been established for lack of evidence.
Both fires came as a city-funded downtown El Centro revitalization project is ongoing.
Wednesday's fire was just outside the construction site of a pedestrian mall on the southeastern corner of Seventh and Main.
The blaze did not affect construction as Pilkington Construction Co. crews were working at the site this morning.
Phil Carr, director of administrative services for El Centro and acting city manager, said this morning he doesn't think the fire will affect the revitalization project.
He added, "This is a terrible loss. We're saddened by these kinds of fires. We don't like loss of property, but we're fortunate there were no injuries in fighting it."
City Councilman Larry Grogan said he arrived at the fire scene minutes after the roof to the theater had collapsed.
"When I drove up there last night and saw the collapsed building … it was like looking at the ribs and skeleton of something that had been great, and it was just in its death throes," he said. "You saw the smoking ruins and it was just such a loss."
El Centro Mayor Cheryl Walker credited the efforts of Valley firefighters, adding her first inquiry when she arrived on scene was into the safety of the firefighters.
She added she is "incredibly disappointed" about the loss of the buildings.
Grogan said of the theater, "It's such a keystone of downtown development."