Traffic on the westbound 60 Freeway was halted in Moreno Valley after a speeding BMW sedan overturned, killing four people inside, officials said.
The white 1999 BMW 328i hit the freeway's center median around 7:30 p.m., causing it to flip over near the Redlands Boulevard exit, said California Highway Patrol Sgt. Eugene Norris.
The westbound lanes were shut down at Redlands Boulevard as a result. At about 11 p.m., officials said they expected the lanes to be closed another several hours. Though just one eastbound lane had been closed, traffic on that side of the road could be seen backed up to the 215 Freeway.
Authorities remained on the scene after 11 p.m., working to collect evidence and clear the freeway, aerial video showed.
Several witnesses told CHP the BMW was racing with another car before it struck the center divider.
The force of the crash caused two of the sedan's occupants to be ejected, while another three were trapped inside, the Riverside County Fire Department said.
Video from the scene showed two bodies blanketed in yellow tarp lying near the center divider, several feet from where the white car came to rest on its head in the fast lane.
Four of the occupants died at the scene, while another person suffered serious injuries, fire officials said.
The victim appeared to be in their early 20s, Norris said, but their genders were unknown.
Investigators were working to determine whether they were wearing seatbelts.
Officials are now searching for two suspect vehicles: a 2017 or 2018 Honda Civic, either gray or black, and a black Mazda hatchback.
Anyone who witnesses the incident is asked to contact the CHP's Riverside Division at 951-637-8000.