WASHINGTON The National Highway Traffic Safety Administration revealed late last month it has begun an investigation of an apparent series of driveshaft failures on D-model MCI coaches operated by Greyhound Lines.

The investigation was launched after Greyhounds parent company, FirstGroup America, filed documents with NHTSA alleging Greyhound had experienced "several driveshaft failures" on D coaches, starting in March 2010.

The filing said that when the driveshafts failed they were not contained by safety loops designed to keep flailing drivelines from possibly damaging other components that could lead to a crash.

In two incidents reported by FirstGroup, "the driveshaft failures resulted in loss of control of the bus and multiple injuries and fatalities."

Specifically, FirstGroup cited damage to the steering lock of the tag axle possibly causing loss of control.

MCI issued a strong statement, following publication of the first news reports of the NHTSA investigation, asserting that the initial media accounts had been inaccurate or misleading.

Saying it wanted to "clarify misperceptions," MCI said "certain articles have inaccurately stated or implied that NHTSA has already made determinations concerning the coaches, when in reality the NHTSA announcement simply relayed what one coach operator has reported to NHTSA, and the fact that NHTSA would be seeking additional information in order to make any determinations.

"MCI welcomes NHTSA's investigation and will provide its full support and cooperation," the company said.

"MCI is proud of the safety and reliability of its D-series coaches, in addition to all of our other models. We are confident that NHTSA will conclude after its investigation has been completed that the claims are without merit."

The driveshaft incidents experienced by Greyhound are baffling in many respects -- and to many people in the industry.

The most serious aspect, of course, is the allegation that crashes are resulting from the driveline failures and fatalities have resulted.

The crash -- where fatalities occurred after a driveshaft failed -- happened mid-morning on March 15, 2010, on Interstate 37, about 14 miles southeast of Pleasanton, Texas. Pleasanton is 35 miles south of San Antonio.

The coach was a 1995 MCI 102 DL3 that was being operated by a Greyhound affiliate, Americanos USA.

It was traveling south on I-37 when the driveshaft broke, causing a loud noise and vibration, according to a "reconstruction investigation" by the Texas Department of Public Safety.

According to witnesses, the driver and a co-driver, who was standing behind the driver, were trying to figure out what had happened to cause the vibration and noise from the rear of the coach.

The driver then "locked the brakes" and the coach slid to the right, reported the investigators. The driver then "overcorrected back to the left," causing the coach to enter into a side skid. The coach eventually slid into the median and rolled onto its right side.

Two people were ejected from the coach during the roll and were crushed to death.

The Texas DPS investigation concluded the driver should have been able to bring the coach to a safe stop despite the driveshaft failure. The investigators said the "heavy application of the brakes by the driver caused her to lose control."

The other incident reported to NHTSA cites driver distraction and error.

The latest instance of a Greyhound coach dropping a driveline occurred last month in Oklahoma City on March 28.

A DL3, that had gone through the extensive Greyhound refurbishing program in Nappanee, Ind., was returning to Oklahoma City from a West Coast trip and was less than five miles from the downtown terminal when it dropped a driveshaft on I-40.

The driveshaft came loose at the transmission end, flailed around, knocking shock absorbers loose from their brackets on the tag and drive axles, pulling hydraulic lines out of the transmission, tearing the driveline yoke to pieces, and knocking the driveline safety loop off the coach.

Debris apparently hit a leveling valve, raising the coach to its full height.

Despite the melee the driver was able to bring the coach to a safe stop.

The incidents are confounding to industry experts for a whole variety of reasons. They cite a busload of possible issues and causes: design, component replacement, maintenance and repair, parts quality, operational issues and other factors.

And, they ask, why are the incidents occurring regularly at Greyhound?

As a rule, driveshafts seldom fail and fall completely off a coach. They, and the u-joints that are integral to them, are robust components but they are subject to lots of stress.

Angle is very important. For example, if there is too much driveshaft angle it overstresses the u-joints, which can fail. If suspension height varies because leveling valves are set incorrectly or fail, then angles can get out of whack, stepping up possible failure.

Maintenance, periodic inspections and keeping bearings greased also are critical.

NHTSA said it will examine whether the buses have a safety defect. Its investigation will cover an estimated 4,000 D models produced between 1992 and this year.

An investigation can lead to a safety recall but no vehicles have yet been recalled.