LANCASTER, Calif. – A mass casualty incident was declared Wednesday morning after a city transit bus carrying multiple passengers, including children, overturned following a crash on railroad tracks in Lancaster, according to the California Highway Patrol (CHP) and Los Angeles County Fire Department.

The crash occurred at approximately 11:18 a.m. near Sierra Highway and West Avenue C.

Initial reports indicate that the bus lost control, crossed over the railroad tracks, and overturned, coming to rest on its side adjacent to the tracks.

Emergency crews responded swiftly to the scene, where they reported as many as 20 individuals requiring medical attention.

The exact number of adults and children on board at the time of the crash has not yet been confirmed.

Among the injured is an 8-year-old boy, who is reported to be in stable condition.

A medical helicopter was requested to transport him for further treatment.

Another individual is in critical condition, though no additional information about that patient’s identity or age has been released.

The City of Lancaster confirmed that the railroad tracks sustained damage as a result of the crash.

Union Pacific and Metrolink, which operate trains on the affected route, have been notified of the incident. It is not yet clear whether any train traffic was active or scheduled in the area at the time.

As of Wednesday afternoon, both directions of Sierra Highway near the crash site have been closed to allow for emergency operations and a thorough investigation by the California Highway Patrol.

Drivers are advised to avoid the area and seek alternate routes.

This remains a developing story.

Authorities are working to determine the cause of the crash and confirm the identities and conditions of those involved.