UPDATED 11:20 a.m. Tuesday with information from relative.

STE. GENEVIEVE COUNTY, Mo. -- A couple from Hillsboro who had traveled for hours on a roadtrip to Texas were killed in a rollover crash on Interstate 55 as they were nearing home.

Casey L. Mendenhall, 26, and her husband, Trenton Mendenhall, 28, died about 11:15 a.m. Monday on I-55 in Ste. Genevieve County. They were the parents of three children -- but the kids didn't accompany them to Texas and weren't involved in the crash.

Trenton Mendenhall was driving a 2010 Volkswagen Beetle north on I-55 when his car veered off the left side of the road at Little Saline Creek Bridge. The Missouri Highway Patrol said his car drove into the median, traveled between the bridges, vaulted down an embankment and landed on its top.

The patrol's report does not indicate why the car went off the road.

But Trenton Mendenhall's mother, Sharilyn Mendenhall, believes her son must have fallen asleep at the wheel. She said her son and daughter-in-law were returning from visiting friends in Texas when they crashed. After driving for hours, "they were just 25 miles from home," she said, when the crash happened.

Sharilyn Mendenhall said she had warned him on his Facebook page earlier not to drive too long without sleep. "I had told them that if they get tired, they should pull over and take a nap," she said.

Both Casey and Trenton Mendenhall were pronounced dead at the scene by the Ste. Genevieve County coroner, Leo Basler. The patrol said both had been wearing seatbelts.

Casey and Trenton Mendenhall are survived by three children: a 10-year-old daughter and sons, ages 11 months and 3 years old. Trenton's mother said her son worked as a roofer. "He was a happy kid," she said.

The couple's three children did not accompany them on the Texas trip, so they weren't in the car when it crashed. The children are now staying with relatives.