A federal safety board concluded yesterday that a fatal June 1998 Greyhound bus accident on the Pennsylvania Turnpike was probably caused by a fatigued driver who took antihistamines and lost alertness before crashing into a parked tractor-trailer, killing himself and six passengers.

In its finding of probable cause in the 4:05 a.m. accident that also injured 16 other passengers and two occupants of a truck, the National Transportation Safety Board was also sharply critical of Greyhound Lines Inc., which the board said failed to detect and correct widespread speeding by drivers and disregarded anonymous calls to an 800 number reporting unsafe driving.

Dallas-based Greyhound, which carries about 23 million passengers on routes nationwide, vehemently disagreed, saying that its investigation found the 11-year veteran driver pulled off the road when he suffered "either a heart attack or a severe angina attack."

The company also defended its safety record, saying the Transportation Department found last July that Greyhound had an accident rate of 0.55 accidents per million miles, or about a third the rate for all commercial vehicles.

The NTSB said the Pennsylvania Turnpike Commission's practice of routinely permitting nonemergency parking in such areas also contributed to the accident.

Although Greyhound said that two pathologists studied tissue from driver Scott Wisner's body independently and concluded that cardiac failure was the cause of the accident, the NTSB said that "the evidence cannot establish that the bus driver was impaired or incapacitated by a cardiac condition prior to the accident." The board said that the driver took antihistamines "at some time either shortly before or during his scheduled route."

It also said that "because of the scheduled irregular work-rest cycle and possible sleeping difficulties, the bus driver may have developed a sleep debt over the four days of his shift." Greyhound said, however, that Wisner was on the second day of a regular schedule that he chose and worked routinely.

On that schedule, said Greyhound spokesman George Gravley, Wisner worked four days driving two round trips from Pittsburgh to New York, had two days off and then repeated the four-day run.

The NTSB said it had clocked Greyhound buses exceeding the speed limit, including one going 73 mph in a 45 mph zone, during its investigation.

Gravley said the company has improved its programs to control speeding as a result of that information. He said that Greyhound does not disregard anonymous calls to the 800 number, but cannot use anonymous calls to discipline drivers.

If Greyhound finds a pattern of complaints, however, he said that it has other means to monitor drivers, including supervisors who follow buses and quality control staff who ride buses anonymously.