WEST, Texas — After spending the night sifting through the debris left by the devastating explosion at a fertilizer plant, authorities Friday raised the number of dead to 12, most of them firefighters and other emergency responders.

“We’re still in search-and-rescue mode,” Sgt. Jason Reyes of the Texas Department of Public Safety said at a news conference.

About 200 people were injured by the blast, which tore apart an entire section of West, a small town of roughly 2,800 residents 80 miles south of Dallas. By daybreak Friday, rescue personnel had combed through 150 buildings, though there were 25 more to go. Fifty homes were completely destroyed, as well as three firetrucks and one ambulance, Reyes said.

No one among the hundreds of local, state and federal officials and first responders who converged on this town north of Waco was certain about the cause of the disaster.

A spokesman for the FBI in San Antonio said Thursday that there had been no indication of criminal activity in the West plant explosion.