DALLAS, Texas (CNN) -- Investigators want criminally negligent homicide charges brought against the driver of a bus that caught fire and exploded outside Dallas, killing 23 nursing home patients who were fleeing Hurricane Rita, a sheriff's spokesman said Monday.The driver, Juan Gutierrez Robles, is currently in federal custody on immigration charges, Sgt. Don Peritz told CNN."We are recommending prosecution on 23 counts of criminally negligent homicide, one for each of the decedents," Peritz said. "It's now up to the district attorney's office to forward that information to a grand jury."Each charge is a felony punishable by up to two years in prison under Texas law, and Dallas County Sheriff Lupe Valdez said an investigation by local, state and federal agencies could yield additional charges.The bus was carrying patients from a nursing home in Bel Air, Texas, near Houston, on September 23, as residents of the Texas Gulf Coast evacuated their homes ahead of the hurricane. The bus caught fire near Wilmer, on the southern outskirts of Dallas, and the blaze caused oxygen canisters brought for the nursing home patients to explode, destroying the vehicle, investigators found.Robles, 37, of Pharr, Texas, and 14 patients escaped the burning bus. The driver was initially credited with rescuing some of the passengers, but Valdez said police "haven't been able to find anyone to confirm that he actually helped someone off the bus."Peritz said the recommended charges were based on "the totality of his actions from the time he left Bel Air until the time the bus caught fire outside Wilmer.""Everyone on the bus was under his care and control," he said.Investigators are still trying to determine who was responsible for the vehicle's maintenance and safety, Peritz said. The bus was owned by a Canadian company and leased to a firm in the United States, which leased it to another company that then contracted with the company Robles worked for -- Global Limo, of Pharr, Texas."We've got to backtrack all the way back to the original bus owners and see who owns what, who leased what to whom, when did they do that, who maintained the bus and how did they maintain the bus," Peritz said.Global Limo, which was shut down by federal officials this month, had no comment Monday, according to The Associated Press.