TULSA, Okla. (AP) - Federal investigators say the woman accused of driving her car into spectators at Oklahoma State University's homecoming parade and killing four people sped up as she approached the parade route.

The National Transportation Safety Board said Thursday that co-workers who saw 26-year-old Adacia Chambers before the crash said she seemed distracted. The NTSB said her "emotional distress" was the probable cause of the crash on Oct. 24, 2015.

"When confronted by her manager about her behavior, the driver became emotionally distressed, made reference to quitting her job, and ran to her car. Given that the crash occurred only about 10 minutes after the driver left her job, she was most likely still experiencing emotional distress when the crash occurred."

The report says Chambers' car accelerated from 54 mph to 59 mph in the five seconds before she struck a police motorcycle blocking the roadway.

"Despite the driver’s having a clear line of sight and sufficient distance to stop, no braking or steering input was recorded prior to the impact with the motorcycle," the report states.

The NTSB says she applied the brakes after hitting the motorcycle, though the report doesn't say how fast she was driving when she hit the crowd.

Chambers has pleaded not guilty to four counts of second-degree murder.