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. Spokesman Keith Holloway said the agency will look at the operations of the charter bus company as part of its investigation and has requested inspection and maintenance records.

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 been interviewed by police.

The bus crashed Saturday north of Laredo.