OAKLAND -- Four people, including an 8-year-old Oakland boy and his parents, were killed when their car collided with a minivan just outside Placerville over the weekend, authorities said. Three people were injured in the crash.

The crash happened Saturday night when a 2004 Toyota Prius traveling west on Highway 50 crossed the center divider, striking a 2011 Toyota Sienna minivan going east, said Officer Quinn Cuthbertson, a California Highway Patrol spokesman.

As of Monday, investigators had not determined what caused the Prius to cross into oncoming traffic.

The driver of the Prius, Sheng Kun Su, 38, his 39-year-old wife, Shu Zhen Zhao, and their 8-year-old son, Shawn Su, all from Oakland, died in the crash. A fourth passenger, the couple's 10-year-old daughter, Elly Aili Su, survived with moderate injuries, said El Dorado County Sheriff's Office Sgt. Jim Byers.

She was taken to Marshall Hospital for treatment of her injuries and later released to extended family in the Bay Area.

A 4-year-old passenger in the van -- Nora Elizabeth Temple, of Pollock Pines -- also died.

Her mother, Danielle Marie Temple, 35, who drove the van, and her father, Will John Temple, 51, suffered minor to moderate injuries, Byers said. The couple's 1-year-old twin boys also were riding in the van but were not injured, Cuthbertson said.

Will Temple remained hospitalized Monday in fair condition at Sutter Roseville Medical Center in Roseville, said Detective Jeramy Buckman.

Temple's stepbrother, Oakland resident Peter Ferioli, called the situation a tragedy.

"They'd just gone shopping at Whole Foods and were heading home," Ferioli said. "They're holding up as far as can be expected. Thank God the twins came out unscathed."

He said he was told an injured deer was found in the roadway after the crash, but Cuthbertson could not confirm that claim.

"All the children were in safety seats, and everyone was buckled," Cuthbertson said Monday.

Alcohol was not believed to be a factor in the wreck, but the crash remained under investigation, pending the results of toxicology tests, Cuthbertson said. The results could take up to eight weeks, officials said. Autopsies were set for Tuesday.

The weather was clear at the time of the crash.

Investigators have pulled computers from both vehicles that would indicate how fast each was going at the time of the crash, but collecting that information would probably take days, officers said.

Highway 50 at that location has a speed limit of 65 mph.

"The vehicle will be inspected by (the CHP) to determine if there were any mechanical issues, although at this time we don't believe that was the issue," Cuthbertson said.

Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund.