(CNN) -- The driver of a tractor-trailer who passed a tour bus before it crashed denies the two vehicles clipped each other, as the bus driver has claimed, according to a law enforcement source involved in the investigation.
"We're treating him as a witness, not a suspect," the law enforcement source told CNN, referring to the truck driver.
The source said police tracked down the tractor-trailer driver hours after the accident early Saturday that resulted in the deaths of 15 people and injuries to 17. The 15th death occurred Monday, a New York official told CNN.
The bus driver had told police that he lost control of the bus after swerving to avoid the tractor-trailer and that the two possibly hit each other, police said.
The tractor-trailer's driver says it didn't happen, according to the source.
Some passengers told CNN affiliates that it appeared the driver fell asleep at the wheel and they felt the bus hit rumble strips before the crash. Those strips are generally meant to warn a driver that they have drifted onto the shoulder of the highway.
National Transportation Safety Board Vice Chairman Christopher Hart said he is aware of those reports.
His investigators have interviewed only two passengers so far, he said. They also plan on talking with the bus and tractor-trailer drivers. A safety board team also will examine both vehicles for any physical evidence that they came into contact before the crash.
Safety board investigators visited the crash site Sunday. Hart said skid marks indicate the World Wide Tours bus hit the guardrail on the right of Interstate 95 "at least three times" over the course of 480 feet before crashing into a sign post that sliced through the bus.
The bus engine control module, sometimes called a "black box," is on its way to the safety board's lab in Washington for analysis. Investigators hope it will help tell them how fast the bus was going and reveal other data about the crash.
Authorities also plan to examine a forward-facing camera that was on the bus to see whether it captured any images.
Safety board investigators also will look for any evidence of mechanical problems, such as possible steering and brake issues, and several other areas, including driver fatigue and training, and a complete review of the bus company's records.
CNN's Chris Kokenes contributed to this report.