HAYWARD, Calif. (KRON) — A motorcyclist is dead after being struck by a big rig truck on I-880 on Monday, California Highway Patrol confirmed to KRON4. CHP is still searching for the truck involved in the crash.

CHP officers responded to southbound I-880 just north of A Street in Hayward at 11:55 a.m. for the crash. CHP’s preliminary investigation indicates that the motorcycle went down on the highway before its driver was struck.

CHP does not know what caused the motorcycle to go down. It is also unclear if the driver of the truck knew that they were involved in an accident.

A sig alert was issued after the crash and four lanes were closed. All lanes were back open again around 2:45 p.m.

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Police have spoken to one witness but are searching for more. Anyone with information about the crash is asked to call CHP’s Hayward office at (510) 489-1500.