The crash happened on the southbound 5 Freeway at the southbound 14 Freeway connector just after 3 a.m. It began with a dump truck that lost control and crashed, causing four big rigs to crash behind it.
Big rig driver Angel Flores said he only had seconds to react, as he drove out of the tunnel and saw four rigs piled up in front of him.
"Inside the bridge, there's lights, but when you come out, it's total darkness ... all of a sudden, whoa, there it is," he said.
Three big rigs were able to avoid getting into a pileup of their own, but behind those trucks were an additional five big rigs that collided, creating the second accident.
A driver was trapped for more than two hours in the cab of one of the trucks. He was extricated and taken to the hospital with major injuries, but was said to be in stable condition.
About 40 gallons of diesel fuel spilled in lanes. Official said it was not enough to cause significant concern for a hazmat situation, but it took several hours for crews to clean up the mess. The truck route reopened at about 4:15 p.m.
The cause of the crash remains under investigation. Authorities are trying to determine if speed and slick roads due to light rain contributed to the pileup.
An accident in the same tunnel in 2007 killed several drivers and shut down the roadway for a month.
Correction: It was earlier reported that hundreds of gallons of fuel spilled onto lanes.