The bus driver told police "nothing happened" when he hit the brakes on I-405 in Kirkland on the night of May 6, but inspectors weren't able to find any evidence to back up that claim.
Robert and Elizabeth Rotta were killed when the bus slammed into their SUV. Their son Ken, who was driving the SUV, survived.
"There was a big square bus and I didn't even have time to look at it. I saw it, boom!" Ken said after the crash.
Since then the state patrol has been investigating the incident, trying to determine if the bus driver's statement about the brakes failing was true.
Witnesses said the bus went through a red light just before the crash, and the bus finally came to a stop 3,000 feet after the crash site.
A mechanical evaluation of the bus included tests of braking, acceleration and steering systems and showed no mechanical issues, according to WSP.
A routine inspection several weeks prior to the crash showed the bus pulling to the right, but during a test drive the second mechanic found no problem.
Witnesses at the time of the crash weren't sure what could have caused it.
"I feel so bad for those folks, because honestly there's no way this should have happened, whether it was a malfunctioning bus or whatever the case may be. Those folks didn't deserve what they got," one witness said.
Detectives are now using what's called a human, vehicle, environmental model to determine the cause of the crash. Troopers say that could take several months to complete.
The bus driver, who has not been charged with any crimes, is on unpaid leave.