University of California, Irvine Medical Center spokesman John Murray said Adenhart died after undergoing surgery.
Adenhart, 22, was one of three people killed in the crash just after midnight local time, after pitching in a loss against the Oakland Athletics. He got a no-decision after six scoreless innings, giving up seven hits, striking out five and walking three in just his fourth major league start.
The driver of a minivan ran a red light in Fullerton and hit two cars, including one carrying Adenhart.
Police Lt. Craig Brower said the minivan driver fled the crash scene and was captured a short time later. The driver was arrested and booked for investigation of felony hit-and-run.
A fourth passenger was at the same hospital in critical condition.
According to the Los Angeles Times, two people were pronounced dead at the scene, and two were taken to Irvine Medical Center.
Adenhart, a 6-foot-3, 185-pounder from Silver Spring, Md., was a 14th-round pick in the 2004 draft, and made his major league debut on May 1, 2008.
He has spent most of his time since with Triple-A Salt Lake, but got his break this year with a good spring training and the fact the team needed help in the starting rotation, with John Lackey, Ervin Santana and Kelvim Escobar all starting on the disabled list. Information from The Associated Press was used in this report.
The kid pitched 6 scoreless innings last night and just like that he is gone.. Very sad story to cover… My condolences go out to his family and friends as well as the entire Angels organization. -TO