UPDATE: Blown tire on bus was a retread

SHERMAN, Texas - Investigators say a blown tire on a Missouri-bound charter bus that crashed and killed 15 people had been refitted with a new tread in violation of safety standards.

The National Transportation Safety Board also said late Friday that the driver was a 52-year-old who had a commercial license but whose medical certification had expired. Officials also said the bus was operating illegally, without a federal license to operate a charter.

Authorities say the bus was traveling at about 12:45 a.m. when the right front tire blew. The vehicle smashed into a guardrail and tipped over.

Officials say 12 people died at the scene and three others at Dallas-area hospitals.

The bus was carrying a Vietnamese-American Catholic group from Houston to the annual Marian Days pilgrimage in Carthage.

The Marian Days pilgrimage, begun in the late 1970s, attracts thousands of Catholics of Vietnamese descent and includes a large outdoor Mass each day, entertainment and camping at night.