The crash was reported at 1:48 a.m. on the northbound Ventura Freeway south of White Oak Avenue, according to the California Highway Patrol.
The initial call was about a black sedan blocking the fast lane with its hazard lights on, the CHP said. A person was reported out of the vehicle within a few minutes.
Witnesses said four vehicles were involved in the crash, and one person was under a vehicle after the collision.
Video from the scene showed Los Angeles Fire Department firefighters using a jack to raise the wreckage of a car off of a motionless person and check for vital signs.
A representative from the Los Angeles County medical examiner's office was summoned to the scene, the CHP said.
A SigAlert was issued at 2:38 am., shutting down all northbound lanes and diverting traffic off at White Oak Avenue.
The CHP later reopened two lanes, while three others remained closed amid the investigation.
The cause of the crash was unknown.