WASHINGTON Negligence, disregard for safety laws, bureaucratic inertia, and a tragic sequence of events lead to the death of 17 Vietnamese Catholics making a motorcoach pilgrimage from Houston to Missouri last year, a federal investigation has concluded.

A punctured front tire, resulting in a slow leak, was the direct cause of the bus crash near Sherman, Texas, investigators found.

The National Transportation Safety Board, meeting here last month, formally adopted the report of its investigation of the tragedy as survivors of the accident and relatives of the passengers who were killed looked on.

The report contains 24 conclusions and a dozen new recommendations. It also reiterates five previous safety recommendations.

The NTSB said the Aug. 8, 2008, crash and high death toll were caused by three things: a damaged front tire that failed, a guard rail on a low bridge that was neither substantial nor high enough to keep the bus from veering off the road and toppling over, and the lack of safety protection for the occupants.

The bus was owned by a rogue Houston charter operation that had been ordered closed less than two months earlier by a federal safety inspector.

After the board voted to accept the report, Chairman Deborah Hersman called on federal regulatory agencies to enact the safety measures included in the report, as well as actions cited in previous reports involving interstate motorcoaches.

"We know the standards can be improved," said Hersman, citing items that have long been on the agency's "most-wanted" list of highway safety reforms. "I want those recommendations closed in a positive way and taken off the most-wanted list."

The three members of the NTSB voted unanimously to attribute the crash to the under-inflated tire after reviewing results of an investigation that began when the coach, its driver and 55 passengers crashed through a bridge railing on U.S. 75 en route to Carthage, Mo.

The board determined that "the probable cause of this accident was the failure of the right steer-axle tire due to an extended period of low-pressure operation which resulted in sidewall belting and body ply separation within the tire, leading to loss of vehicle control."

The safety board said the "failure of the bridge rail to redirect the motorcoach to prevent it from departing the bridge" contributed to "the severity" of an accident that injured the driver and 38 other passengers. The bus was traveling at 68 mph at the time.

"The inadequate occupation protection system (onboard the bus) contributed to the severity of the passenger injuries," the board added, recommending that the Federal Motor Carrier Safety Administration urgently consider safety measures in long-distance buses such as seatbelts and stronger seats.

The board concluded that the driver of the bus, operated by Iguala BusMex, used cocaine and alcohol before the trip and "may have been impaired." But the board said there was "no evidence that the actions of the driver caused or contributed to the tire failure" or the series of steps that claimed passengers' lives after the bus fell about 8 feet off the bridge and slid roughly 24 feet on its right side before coming to rest.

"Even a well-rested, completely alert driver could not have reacted in time to affect the sequence" of events that followed the failure of the tire at highway speed, leading to the accident, the board said.

The tire was a retread. Using retreaded tires on the steer axle of a motorcoach violates federal rules, but the NTSB decided that did not cause the tires failure. The bus passed inspection at a state-certified facility in Houston eight days before the accident, when an inspector did not notice the retread.

Iguala was placed "out of service" by the Federal Motor Carrier Safety Administration and remains closed.

Owner Angel de la Torre, who has been sued by victims and their relatives, could face charges now that the NTSB report is completed.

Major safety issues identified by the accident investigation include the following:

The tire failure and the need for tire pressure monitoring systems on vehicles weighing more than 10,000 pounds.

The lack of oversight of the federal commercial vehicle inspections that are delegated to the states.

The lack of motorcoach occupant protection systems.

The deficiencies in federal safety oversight of new entrant motor carriers.

As a result of its investigation, NTSB made several recommendations. One recommendation was directed to the American Association of Motor Vehicle Administrators regarding tire pressure monitoring procedures, and one recommendation was made to the coach manufacturer, Motor Coach Industries, calling for revised vehicle maintenance manuals.

NTSB also reiterated five previous recommendations made to the Federal Motor Carrier Safety Administration and the National Highway Traffic Safety Administration in the areas of drug and alcohol testing and motorcoach occupant protection systems.

A synopsis of NTSB report, including the probable cause and recommendations, is available on its website, www.ntsb.gov, under "Board Meetings."