SHERMAN, Texas (AP) -- A charter bus carrying a Vietnamese Catholic group to an annual pilgrimage slammed onto its side and then skidded off a freeway early today in North Texas, killing at least 13 people and injuring more than 40.

The right front tire of the bus carrying 55 people from Houston to Missouri blew out, although officials had not determined whether that was the cause of the wreck.

Fire Chief Jeff Jones said 10 people were airlifted to hospitals and the rest of the passengers were taken to hospitals by ambulance, many with serious injuries.

The bus smashed into a guard rail at about 12:45 a.m. on a bridge that's about 15 feet above a creek, apparently skidding along the guard rail before sliding off U.S. 75 on the other side of the bridge.

The bus came to a rest on its right side, partly on the northbound lane of the freeway and partly on the grass. Workers righted the bus early today and loaded it onto a large flatbed truck.

A Galveston/Houston archdiocese church official said many of those on the bus were from the Vietnamese Martyrs Church of Houston and were on their way to a religious festival honoring the Virgin Mary in Carthage, Mo.

Sherman police Lt. Steve Ayers said investigators have interviewed the bus driver and he is in stable condition. It is not believed that alcohol was a factor and it's too early to tell if fatigue played a role in the crash, Ayers said. Roads were dry at the time of the wreck.

The National Transportation Safety Board is investigating the wreck.