Federal regulators on Friday ordered a tour bus operator involved in a Southern California crash that killed eight people to immediately stop operating because its buses weren't properly maintained or inspected and its drivers weren't properly vetted for qualifications.

The Federal Motor Carrier Safety Administration said in a statement that it ordered all three buses operated by National City, Calif.-based Scapadas Magicas LLC in the U.S. to stop operating because they pose an imminent hazard to public safety.

The roadworthiness of the 1996 bus involved in the Feb. 3 crash has been in doubt after the driver said the brakes failed before it smashed into a sedan, flipped, slammed into a pickup and crashed on a road in the San Bernardino Mountains east of Los Angeles. The bus was carrying 38 people, including the driver and a tour guide, to Tijuana, Mexico.

Regulators said Friday that a post-crash investigation of the company's two other buses that had been operating in the U.S. found serious mechanical safety violations.

Inspectors said the company failed to have its vehicles regularly inspected before the deadly crash. When the buses were inspected, there were many violations, including multiple brake problems.

Vehicle maintenance violations were cited during 21 of Scapadas Magicas' 25 most recent vehicle inspections, and vehicles had to be placed out of service after 36 percent of the inspections, according to the order to cease operations.

In a January 2013 compliance review conducted by transportation regulators, breakdowns were identified in the company's safety management systems, but "Scapadas Magicas failed to take necessary action to remedy its safety management deficiencies," the order said.

Calls to Scapadas Magicas' offices seeking comment Friday night were unsuccessful.