Boston Celtics star Isaiah Thomas's sister Chyna Thomas was killed Saturday in a single-car road accident, the Tacoma News Tribune reported.
The Celtics confirmed the death with the release of a statement later in the day.
"We are terribly saddened by the tragic loss of Chyna Thomas," the NBA team said. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
The Washington State Patrol said Chyna Thomas, 22, died at the scene of the accident at around 5 a.m. in Federal Way, Washington, located between Tacoma and Seattle.
Witnesses told authorities that her car was moving normally before drifting into a barrier then hitting a post head-on.
Isaiah Thomas was instrumental in Boston's emergence as the top seed in the NBA's Eastern Conference playoffs.
It was not immediately clear if he would be available for the team's first-round series opener against the Chicago Bulls in Boston on Sunday.