The Federal Motor Carrier Safety Administration shut down 26 bus operations running passenger service on I-95 between New York City and Florida, the latest in a series of enforcement crackdowns on truck and bus companies.

The agency said the bus companies were an "imminent hazard" due to continuous safety violations, such as using drivers who did not have commercial licenses and who did not clear drug and alcohol tests.

This is the largest single shutdown in the agency's history, said Transportation Secretary Ray LaHood.

"These aggressive enforcement actions against unsafe bus companies send a clear signal: If you put passengers' safety at risk, we will shut you down," he said in a statement.

FMCSA recently has been nearly as aggressive against trucking operations it found in violation of the rules.

Since March, it has ordered a half-dozen freight carriers to stop service:

- U&D Service, Indianapolis, for using drivers who did not have a CDL and who were not proficient in English.

- Reliable Transportation Services of Utah for hours of service and other violations.

- J&A Transportation of New Jersey for hours violations, as well as driver qualifications and maintenance.

- Judson Mobley Logging, Georgia, for drug and alcohol violations, driver qualifications and maintenance issues.

- BM&L Trucking, Alabama, for hours violations, as well as driver qualifications and maintenance.

- Denny Mekenye, doing business as Demco Express in Texas, for hours, drug testing and driver violations.

The bus shutdown targeted three primary companies, Apex Bus, I-95 Coach and New Century Travel, that oversaw a network of other carriers. The order covers one ticket seller, nine active bus companies, 13 companies that had already been ordered out of service but were continuing to operate, and three more companies that were applying for authority, the agency said.

All together, the companies provided curbside service to some 1,800 passengers a day.

The agency said it has been working on the bus investigation for a year, since a series of fatal crashes last spring.