The incident occurred shortly after 1 a.m. Fire officials said a pickup truck was traveling northbound up Walnut Street when the driver lost control and slammed into a tree.
Six people were inside. Three men and one woman died at the scene.
Officials said alcohol and speed played a part in the crash.
Killed in the crash were:
David Huizar Jr., 22, of La Habra
Jimmy Alonzo Gonzales, 22, of Buena Park
Paul Romero, 25, of Fullerton
Delora Bravo, 22, of La Habra
Destiney Mendoza, 25, and Rochelle Romero, 21, both of La Habra, were transported to area trauma centers with critical injuries.
Witnesses said it was a horrific scene.
"There was a guy hanging out the door," said Adriana Castillo. "He was trying to move, but they were his last breaths."
"We tried to get the female passenger out of the front right seat, but the handle ripped straight off the truck," said another witness, Thomas Snider. "At that point, I ran and got a tool to pry the door open, but nothing worked. We had to wait for the cops to come."
Friends and family members of the victims left flowers and candles at the scene.
A memorial fund was set up at Wells Fargo Bank.
City News Service contributed to this report.