CAMPBELLTON, Texas — Investigators said a broken drive shaft likely caused a bus to veer off a Texas highway, killing two passengers and injuring 40 other people.

Texas Department of Public Safety spokeswoman Tela Mange said today a preliminary investigation shows the drive shaft came apart and fell off the Americanos USA bus as it was driving Tuesday morning on Interstate 37 about 45 miles from San Antonio.

The driver lost control of the bus, and it spun into the center median and flipped onto its side.

A male and female passenger were killed, and two passengers remained hospitalized in critical condition today.

Americanos USA, a subsidiary of Dallas-based Greyhound Lines, has a good federal safety record.

Before Tuesday, the company's vehicles were involved in 10 accidents in three states in the last 30 months, according to Federal Motor Carrier Safety Administration records. Tuesday's crash was the company's fifth involving injuries and second involving fatalities. One person died in a January 2009 crash in San Diego involving one of its vehicles. The company's driver was not cited.

Bonnie Bastian, a spokeswoman for Greyhound's parent company, FirstGroup America, said the company is assisting with the investigation.