Federal transportation officials have shut down 26 bus operators in the Northeast corridor after a year-long investigation found that the companies posed an "imminent hazard" to public safety.

The Department of Transportation, which oversaw the investigation, called the shutdown orders the "largest single safety crackdown in the agency's history" in a press release issued Thursday.

The action applies to nine active bus companies, 13 companies that had been ordered out of service but continued to operate and three companies that had applied to become operators, as well as one ticket seller. The companies were primarily run by Apex Bus, Inc., I-95 Coach, Inc. and New Century Travel, Inc. and collectively transported more than 1,800 passengers daily from New York to Florida along Interstate 95.

The carriers had multiple safety violations, according to safety investigators. The companies employed drivers without valid commercial driver's licenses, failed to create drug and alcohol testing programs and operated buses that had not been routinely inspected and repaired.

The investigation began last year after a series of deadly bus crashes along I-95.

Related: Deadly bus crashes highlight safety issues

The companies cited in the investigation will not be allowed to operate under another name in the future under a new rule that revokes the carrier's operating authority and matches new companies to ones previously shut down.

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