Officers on foot and horseback, police dogs and helicopters, are still searching for a Texas prisoner who stabbed the driver of his transport bus and escaped on Thursday. The inmate is serving two life sentences for murder and attempted murder.

Gonzalo Lopez, 46, was convicted of murder in 2006 for the drug cartel-related killing of a man near the Mexican border. He was later sentenced for attempted murder during a 2004 car chase in Webb County.

Investigators are trying to learn how Lopez managed to remove his handcuffs, access the armed driver behind a metal door, stab the driver, and take control of the bus, while two armed prison officers were on the bus, according to prison officials.

The transport bus was traveling from Gatesville to Huntsville with 16 prisoners. Lopez escaped in Leon County, an area between Houston and Dallas. 

The driver of the bus was reported to be in stable condition, according to Jason Clark, chief of staff for the Texas Department of Criminal Justice.

A $22,500 reward was offered for any clues leading to Lopez’ capture, but was later increased to $50,000 after the manhunt came up with no clues.