The reward has grown to $50,000 for the arrest of a Texas inmate serving life for murder who managed to break free from his shackles, overpower a bus driver and escape from custody, officials said.

Gonzalo Lopez, 46, was on a transport bus en route from Gatesville to Huntsville for a medical appointment when he escaped in Leon County on Thursday, the Texas Department of Criminal Justice said.

Two officers were on the bus: one at the front and one in the back who was armed with a shotgun, Texas Department of Criminal Justice spokesman Robert Hurst told reporters.

Lopez "was somehow able to get out of his shackles and get into the driver's compartment of the bus," Hurst said.

Lopez cut out the bottom of a door that separates inmates and the driver, Hurst said.

"He used some type of device, we don't know what some type of device, to cut out the bottom of the door," he said.

Lopez "was able to overpower the driver. There was a struggle...the bus went off the roadway," Hurst said.

Lopez allegedly tried to grab the driver's service weapon but couldn't remove it from the holster, he said.

During their fight, the officer driving the bus was stabbed in the hand and punctured in the chest, suffering non-life-threatening injuries, Hurst said.

The officer in the back of the bus fired two shots into the rear wheels of the bus, he said. Lopez was able to drive the bus for about 1 mile with the flat tires before he crashed, Hurst said. Lopez then jumped off the bus and fled, Hurst added.

Shots were fired at Lopez as he fled across a cow pasture, Hurst said.

The other inmates remain on board, he said.

Lopez is serving a life sentence for a capital murder in Hidalgo County and an attempted capital murder in Webb County, the Texas Department of Criminal Justice said. The murder was committed with a pickaxe, Hurst said.

"We do not know if he has obtained any kind of a weapon," Hurst said. "Last we saw him he did not appear to have a weapon in his possession, but who knows what he might've been able to get."

Hurst commended the two officers who he said "acted very heroically...The officer that fired the shots with a shotgun into the rear wheels of the bus, that was smart thinking."

Centerville School District schools are closed on Friday as the search continues.

Leon County is about 130 miles south of Dallas. The Leon County Sheriff's Office urged local residents to lock their homes and cars. Anyone who sees Lopez is asked to call 911 and not approach him.

A $15,000 reward initially had been offered for information leading to his capture, the Texas Department of Criminal Justice said. The reward increased to $22,500 Friday night, then more than doubled to $50,000, the department announced Saturday night.

The Texas Department of Public Safety also added Lopez to the state's 10 Most Wanted Fugitives List on Friday, noting that he has ties to the Rio Grande Valley and San Antonio and is allegedly affiliated with the Mexican Mafia.

Officers expect rising temperatures in the area this weekend will help them find Lopez.

ABC News' Jennifer Watts contributed to this report.