ATASCOSA COUNTY, Texas -- A man and a woman were killed and dozens were injured when a Mexican tour bus crashed Tuesday morning in Atascosa County.

Police said 28-year-old Efrai Dominguez-Valenzuela and 57-year-old Christian Campos were killed in the accident.

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Tommy Cavazos, a public communications officer with the Texas Department of Public Safety, said that the Americanos tour bus rolled over near the Campbellton exit off Interstate 37, about 54 miles south of San Antonio at 9:45 a.m.

Two people were airlifted to Brooke Army Medical Center and three were airlifted to University Hospital in San Antonio.

According to a news release, 27 victims were transported to South Texas Regional Medical Center in Jourdanton. As of 6 p.m., 19 had been discharged, four were listed in stable condition at STRMC and four had been transferred to San Antonio metro-area hospitals.

The bus was on its way from San Antonio to Matamoros, Mexico, when it crashed. There were about 42 people in the bus.

The driver of the bus heard a popping sound when she lost control and the bus rolled over. The driver told DPS she thought the popping sound was her tire, but troopers checked out the tires and they were not damaged.

DPS investigators are now looking into a mechanical malfunction.

Some drivers pulled over to help. Carmen Alaniz was among several witnesses who tried to help the victims.

"(We) started helping people out of the bus," she said. "People crawling out the window and underneath trying to get out. We did what we could to stabilize people."

Mark Baldon, who came upon the crash about 15 minutes after it happened, told KSAT 12 News that the bus was on its side in the median.

"(The) bus was on its side and it was smoldering," Baldon said.

According to DPS, most of the injured suffered cuts, abrasions and fractures. DPS trooper Jason Reyes said the passengers were a cross-section of men, women and children, typical of those traveling to the border to visit family and friends.

Americanos set up a hotline for relatives of passengers needing information on the crash. The number is 1-800-972-4583. South Texas Regional Medical Center also set up a patient information line at 830-769-5230. Family members will need to provide the name of the victim in order to receive an update on the patient's condition.

Americanos USA is a subsidiary of Greyhound bus lines.

The bus that crashed Tuesday was built in 1995.

The driver's name hasn't been released but she has been an employee of Americanos since May 2009 and has a clean driving record.

Americanos buses have been involved in three crashes in Texas in the last three years, but none of those involved fatalities. Two of the three crashes were in Bexar county, but neither had any injuries.

Americanos USA spokesman Timothy Stokes said "our hearts go out to (the victims') families and friends at this difficult time."