It occurred at 3:09 a.m. between Santa Fe Drive and Manchester Avenue when a motorist in a Mercedes made a U-turn at the border after trying to cross into Mexico, said CHP spokesman Jim Bettencourt.
The Mercedes then headed north on the southbound I-5 as Border Patrol officers tried unsuccessfully to catch up. Soon afterward, the Mercedes collided head-on with a Chrysler traveling south on the southbound I-5, he said.
One person in the Mercedes and two in the Chrysler were killed instantly, Bettencourt said.
No further information was available, and the investigation is continuing.
Bettencourt said motorists should seek alternate routes through that stretch of I-5, which is expected to be closed for several hours.