CAMPBELLTON — A broken drive shaft likely caused a Mexico-bound bus to veer wildly on a Texas highway before flipping over, killing two passengers and injuring 40 others on board, investigators said Wednesday.

The drive shaft, which transfers power from the engine to the wheels, apparently fell off before the Americanos USA bus carrying 42 people careened from the right lane toward the median and spun wildly, Department of Public Safety spokeswoman Tela Mange said. She said the findings were preliminary, and that the investigation was ongoing.

The crash occurred along Interstate 37 about 45 miles from San Antonio, from where the bus had departed. It was carrying spring breakers and other travelers to the Mexican border city of Matamoros, with planned stops in Falfurrias and McAllen, Texas.

Two passengers were killed — Christina Lozano Campos, 62, of Lewisville and Efrain Dominguez-Valenzuela, 27, of Brownsville — and at least two others remained hospitalized in critical condition Wednesday. The other 38 people on board were either hospitalized in stable condition or had been treated and released.

The driver, 47-year-old Irma Morado, was not cited.

The bus underwent its regularly scheduled maintenance the day before the crash, said Bonnie Bastian, a spokeswoman for FirstGroup America, the parent company of Dallas-based Greyhound Lines Inc. Americanos USA is operated by Greyhound.

She said Wednesday that she was not aware of the crash investigation findings, but the regular inspection covered the engine, tires, brakes and wheels. She didn’t know the age of the bus.

Morado and several passengers described hearing a loud noise before the bus lost control. After it flipped, those who could climbed to safety through shattered windows and an emergency exit.

“I think we did a 180. We flipped and I was out the window,” said Daryl Champagne, a 17-year-old San Antonio high school senior who was on his way to South Padre Island with two classmates on spring break.

He crawled free and helped another man pull people through the emergency exit. Someone inside handed him an infant. Many passengers were bloody or appeared to have broken bones, he said.

Karlo Castilleja, 18, said he was pinned in the bus with three people on top of him and his face in the dirt.

“I was scared when I was pinned down. I couldn’t breathe,” said Castilleja, who worked himself free when the other passengers got their bearings and could move.

The National Transportation Safety Board is not planning to investigate because its initial assessment turned up no new potential safety issues involving the company or crash, agency spokesman Keith Holloway said.

The NTSB has long advocated that motor coaches include seat belts and other occupational safety devices, but the recommendations have yet to be turned into law, in part because of strong lobbying by bus companies.

Americanos USA 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.

With 137 motor coaches in service, Americanos has been involved in one accident for every 13 buses. By comparison, Greyhound has been involved in one accident for every eight buses.

In the last two years, inspectors placed Americanos USA vehicles out of service following 11.2 percent of their inspections, about half the national average of 22.3 percent. Inspectors placed the company’s drivers out of service after only 1.8 percent of inspections, a far lower rate than the national average of 6.6 percent.

For 18-year-old passenger Jacob Medina, the bus trip from San Antonio to McAllen had been routine because his father lives in McAllen, but as he left the hospital, he felt lucky to have suffered only minor injuries and uneasy about getting on the road again.

“I’m afraid to get in the car, honestly,” said Medina.