A faulty tire may be to blame in a Juab County rollover Monday that left two teenage boys and their pregnant mother dead, said Utah Highway Patrol spokesman Cameron Roden.

Seven people were in the Ford Explorer traveling south on Interstate 15 near Mona about 5:45 p.m., when the vehicle rolled off the road, ejecting five passengers and partially ejecting a sixth.

The two teens, estimated to be 13 and 17 years old, died at the scene. Their mother, 38, died Monday night, along with the 20-week-old child she carried, Roden said.

"They tried to save baby but were unsuccessful," he said.

At least two of the woman's children survived the crash, Roden said. A 2-year-old and a 4-year-old were in fair condition. A 19-year-old also was in fair condition, but the teen's connection to the family was unknown, Roden said. A seventh occupant was in serious condition. The group was traveling to their home in Las Vegas, Roden said.

Witnesses said the tread of one tire detached, causing the driver to weave back and forth on the road and lose control, Roden said. Investigators do not know how many of the Explorer's occupants were wearing safety belts.

Meanwhile, one motorist died in a separate crash about the same time near 11200 South in Salt Lake County, Roden said.

A man in his 20s was driving a Honda Civic north on I-15 when a Dodge pickup rear-ended him, Roden said. Both drivers lost control, and four other cars were hit. The driver of the Civic was flown to a hospital, where he died. Three others suffered injuries that were not life-threatening.

Both crashes snarled northbound I-15 as UHP had to clear the road for Lifeflight helicopters to land. Traffic was delayed about a half hour in Salt Lake County, Roden said.