Chyna Thomas, 22, died at the scene of the accident on Interstate 5 in Federal Way, Washington, police said.
A source told ESPN's Jeff Goodman that Isaiah Thomas received news of his sister's accident after he participated in Celtics practice in the afternoon.
Thomas was talking to a reporter after practice when Avery Bradley, a fellow Tacoma native, emerged from the trainer's room and pulled him away. Thomas never returned to engage in other scheduled media requests.
A source close to Thomas told ESPN the plan as of Saturday night is for Thomas to play Sunday evening when the team opens its first-round series against the Chicago Bulls in Boston.
Isaiah Thomas' father, James, told the Boston Herald that his son hadn't decided when he would fly home to Tacoma, Washington, to be with family members.
"It's a crucial time for our family right now," James Thomas told the Herald.
In a statement released Saturday, the Celtics extended their condolences to the Thomas family.
"We are terribly saddened by the tragic loss of Chyna Thomas," the Celtics said in the statement. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
NBA commissioner Adam Silver also released a statement on Chyna Thomas' death.
"The NBA family mourns the tragic passing of Chyna Thomas, and we send our deepest condolences to Isaiah, his family and the Celtics organization during this difficult time," Silver said in the statement.
Police said the accident occurred at around 5 a.m., when Chyna Thomas' car suddenly drifted onto the left shoulder of the highway and traveled about 50 feet before hitting a large metal pole.
Police said Chyna Thomas was not wearing her seat belt.
Isaiah Thomas, who was named an NBA All-Star for the second consecutive year, averaged 28.9 points and 5.8 assists in leading the Celtics to the top seed in the Eastern Conference.
|Brian Babineau/Getty Images|
ISAIAH THOMAS' SISTER KILLED IN ONE-CAR ACCIDENT IN WASHINGTON
The sister of Boston Celtics point guard Isaiah Thomas was killed in a single-car accident Saturday morning in Federal Way, Washington.
According to the News Tribune's Craig Sailor, Chyna J. Thomas, 22, was pronounced dead at the scene of the accident after the 1998 Toyota Camry careened into a barrier after the car drifted into the shoulder.
"That vaulted the vehicle head-on into the sign post," State Patrol spokesman Trooper Nick King said, per Sailor.
According to Washington State Patrol, Thomas was not wearing a seat belt at the time of the accident.
"We are terribly saddened by the tragic loss of Chyna Thomas," the Celtics said in a statement Saturday, according to ESPN.com's Chris Forsberg. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
Thomas and the Celtics are scheduled to open their first-round playoff series against the Chicago Bulls on Sunday at 6:30 p.m. ET at TD Garden.
Adam Himmelsbach of the Boston Globe reported members of Thomas' inner circle will head to Boston on Sunday morning to be with the point guard.
Isaiah Thomas' sister, Chyna, dies in car crash in Washington
Chyna J. Thomas, the younger sister of Celtics’ star Isaiah Thomas, died in a one-car crash on Interstate 5 in Tacoma, Wash., on Saturday morning, Kiro7 reports. The crash occurred around 5 a.m.
According to the Washington State Patrol, Chyna Thomas, 22, died at the scene on the shoulder of I-5. Officiers told Kiro7 that she was not wearing a seat belt.
State Patrol spokesman Nick King said the car was initially being driven normally before it began to drift into the shoulder. The vehicle then went about 50 feet and hit a barrier. The 1998 Toyota Camry hit the sign post head on.