WASHINGTON U.S. regulators’ failure to shut down discount bus operator Sky Express Inc. after finding extensive safety violations was a contributing cause of a fatal 2011 crash, the National Transportation Safety Board ruled Tuesday.

Charlotte-based Sky Express, which operated from Manhattan’s Chinatown neighborhood, was closed days after the early morning May 31, 2011, crash outside Doswell, Va., that killed four. The Federal Motor Carrier Safety Administration missed multiple opportunities before the accident to shut down a carrier that didn’t follow basic safety procedures, the board said.

“It wasn’t just the bus driver asleep at the wheel,” the safety board’s chairman, Deborah Hersman, said Tuesday as the board met to vote on what caused the accident. “If you can’t get the worst of the worst off the road, that is a harsh judgment against the agency.”

The safety board’s findings come as the FMCSA, part of the U.S. Transportation Department, is trying to crack down on unsafe bus operators. The department got expanded legal authority to go after bus lines with extensive safety problems, including stiffer fines, in a transportation policy bill Congress passed June 29.

In June, three North Carolina-based passenger bus companies were shut down in what federal officials called the nation’s largest crackdown ever of unsafe bus operators.

The Sky Express crash was caused by a driver falling asleep and a company that didn’t manage fatigue or keep track of its drivers, the safety board said.

Sky Express was operating after being cited for 204 violations in 94 roadside inspections in the 10 months before the accident, according to records the NTSB made public as part of the crash investigation.

FMCSA had conducted four safety reviews in four years at Sky Express, unusual for an agency that inspects 2 percent to 3 percent of bus companies a year due to limited resources. Sky Express wasn’t flying under the radar, Hersman said.

“The crash we discuss today should never have happened,” Hersman said. “It was entirely preventable.”

The Sky Express crash was the third in a span of 11 weeks along I-95 between Virginia and New York last year. Those crashes sparked a yearlong investigation that resulted in an unprecedented enforcement sweep in May, in which the bus regulator closed 26 companies, many of them operating to and from New York’s Chinatown, as imminent safety hazards.

The NTSB recommended that the truck and bus agency change its rules to force new companies to show that they understand U.S. regulations and have safety practices in place before they begin to operate. Current law allows companies to begin running buses immediately, subject to a follow-up review.

FMCSA should also change its rating system to make it easier to shut unsafe operators, the NTSB said. The board first made that recommendation in 1999.

The four passengers who died in the Sky Express crash were crushed after the bus roof collapsed during the rollover. The NTSB reissued a recommendation that the National Highway Traffic Safety Administration develop regulations for better occupant protection in buses, as well as stronger roofs.

Sky Express didn’t have written safety policies or a driver’s handbook on drug and alcohol use, seat belts and mobile phone use, the NTSB found. Its only criteria for hiring drivers was that the applicant held a commercial driver’s license and was 21 years of age or older, it said.

Kin Yiu Cheung, the driver in the Sky Express crash, was hired in July 2010 with no previous commercial driving experience, according to the NTSB. He was previously employed as a restaurant delivery driver.

When Sky Express executives met with regulators for an audit in March 2011, problems were clear, according to audit documents obtained under the Freedom of Information Act.

One driver worked 11 consecutive days without a rest period, according to the documents. Four of 10 drivers couldn’t understand enough English to identify their employer. An insulin-dependent driver made a 938-mile run without medical clearance. All those infractions violated U.S. law.

Inspectors rated the carrier “unsatisfactory,” meaning it had to close in 45 days unless it could prove it had fixed the problems. It didn’t, the documents show, yet the agency gave Sky Express a 10-day extension. The crash happened during the extension period.

When the FMCSA, which regulates all intercity bus services, ordered Sky Express off the road after the accident, the agency cited the same violations behind the “unsatisfactory” rating, documents show.

Sky Express had also received an “unsatisfactory” rating in 2009, agency records show. The company satisfied the agency that it had corrected the issues and was allowed to stay in business. A 2010 audit resulted in a “satisfactory” rating.

Sky Express charged customers $30 for a one-way trip between Durham and New York.

New York-based General Bus has taken over Sky Express’ former building, in front of the shuttered BJ’s on Independence Boulevard, and put up its own sign. But the new company has racked up its own safety violations, including six speeding citations and eight fatigued driving-related infractions. According to the FMCSA’s website, General Bus ranks worse than 98.6 percent of registered carriers for unsafe driving.

Thirty-three people were killed in 13 bus crashes in the U.S. in 2011, according to data compiled by Washington-based Advocates for Highway and Auto Safety.

Staff writer Ely Portillo contributed.