A medical helicopter transports victims Tuesday from a bus accident on an interstate near Campbellton, Texas. Two people were killed and 40 others were taken to hospitals.

CAMPBELLTON, Texas — The bus had been on the interstate about an hour when the spring breakers and other travelers headed toward the U.S.-Mexico border heard a noise and then felt the bus flip around before toppling on its side in a grassy median.

A jumble of passengers landed on broken glass and one another before they began climbing out through the windows and emergency exit. Two people, a man and a woman, near the front of the bus were thrown, dead on impact, passengers said. The other 40 people aboard the bus were taken to area hospitals Tuesday.

"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.

The Americanos USA bus departed San Antonio on Tuesday morning and was headed to the Mexican border city of Matamoros, with planned stops in Falfurrias and McAllen, Texas.

About 45 miles from San Antonio, the bus driver heard a loud noise before the bus veered from the right lane of Interstate 37. The bus landed on its right side between the northbound and southbound lanes, said Chuck Garris, the emergency management coordinator for Atascosa County.

Bus windows were shattered, and luggage, pillows and purses littered the median.

"People were all sitting on the grass stunned, wondering what happened," Garris said. "It was a mess."

Two of the passengers who survived had critical injuries, while others were in stable condition or had been discharged.

Texas Department of Public Safety officials declined to disclose the identities of the man and woman who were killed because their families had not been notified.

Texas DPS Trooper Jason Reyes said the cause of the accident remained under investigation, but there was no initial indication the driver, 47-year-old Irma Morado, was impaired.

She had a valid license to drive the bus and has not been charged. Garris said Morado helped remove passengers from the wreckage.

Investigators suspect equipment failure may have caused the crash, said DPS spokesman Tom Vinger, though officials said the tires appeared intact.

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, 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.

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