Boston Celtics star Isaiah Thomas' sister, Chyna Thomas, 22, was killed in a one-car accident on Interstate 5 in Washington state on Saturday morning, a person with knowledge of the accident confirmed to USA TODAY Sports.

The person spoke under the condition of anonymity because of the sensitivity of the situation.

According to The News Tribune (Tacoma, Wash.), which cited Washington State Patrol and family friends, Thomas' car, a 1998 Toyota Camry, traveled off the road around 5 a.m. and hit a Jersey barrier, which State Patrol spokesman Nick King said "vaulted the vehicle head-on" into a sign post.

According to State Patrol, Thomas was not wearing her seatbelt at the time of the accident and died at the scene.

The Celtics released the following statement Saturday night:

"We are terribly saddened by the tragic loss of Chyna Thomas. The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."

The Celtics open up Game 1 of their playoff series with the Chicago Bulls at 6:30 p.m. ET on Sunday.

USA TODAY Sports' Jeff Zillgitt contributed to this report.