LAREDO, Texas (AP) -- Federal investigators and local authorities are working to determine what caused a charter bus to crash in far South Texas, killing eight people and injuring 44 others in a one-vehicle rollover.

National Transportation Safety Board investigators arrived Sunday in Laredo and will continue their work Monday. The agency will look at the operations of the charter bus company as part of its investigation, and it has requested inspection and maintenance records and examined the bus and the scene, spokesman Keith Holloway said.

Determining the cause could take months, he said.

Meanwhile, a Texas state trooper said Sunday that the state Department of Public Safety is conducting an investigation separate from the NTSB.

Sgt. Johnny Hernandez said the bus driver remains hospitalized with major injuries and has not yet been interviewed by police.

The OGA Charters bus crashed Saturday north of Laredo. Webb County Volunteer Fire Department Chief Ricardo Rangel told the Laredo Morning Times the bus was heading to a casino in Eagle Pass, about 125 miles northwest of Laredo. Officials say the deceased ranged in age from 52 to 61.

State police have said it was raining Saturday morning but it was uncertain if that was a factor in the crash that occurred just before 11:30 a.m.