BOVINA, Miss. (AP) — Investigators with the National Transportation Safety Board on Sunday were at the scene of a bus crash in Mississippi that killed seven people and injured dozens of others.

They plan to look at how well the tires were maintained, NTSB member Todd Inman said at a news conference late Saturday. The NTSB previously said the crash was caused by tire failure.

The 2018 Volvo bus was traveling westbound on Interstate 20 near Bovina in Warren County early Saturday morning when it left the roadway and overturned, according to the Mississippi Highway Patrol. No other vehicle was involved.

Six people aboard were pronounced dead at the scene and another died at a hospital, according to a highway patrol news release. Thirty-six people were taken by ambulance to hospitals.

The highway patrol did not immediately have an update on their conditions Sunday.

The dead included a 6-year-old boy and his 16-year-old sister, according to Warren County Coroner Doug Huskey. They were identified by their mother. Authorities were working to identify the other victims, he said.

The bus was traveling from Atlanta to Dallas with 41 passengers and two drivers, the highway patrol said.

Inman said the NTSB will also look at the carrier’s safety record and protections for bus occupants.

The bus was operated by Autobuses Regiomontanos. A woman who answered the phone at its Laredo, Texas, office said it was aware of the crash, but she didn’t answer questions or provide her name.

The transit company says it has 20 years of experience providing cross-border trips between 100 destinations in Mexico and the U.S. Its website promotes “a modern fleet of buses that receive daily maintenance,” and offers “trips with a special price for workers.”