The crash happened just before 4:40 a.m. east of Huntington Drive, the CHP said.
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Witnesses reported a vehicle speeding westbound in the eastbound carpool lanes. The vehicle was later described as a Chevy Cruze, the CHP said.
The Chevy Cruze then crashed head-on with a sedan, causing one of the vehicles to overturn.
The wrong-way driver was killed in the crash, and the driver of the sedan was hospitalized with major injuries, officials said.
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A SigAlert was issued at 5:05 a.m., closing multiple lanes - as well as the Huntington Drive onramp - for several hours. Traffic was backed up about a mile to the Huntington Drive exit.
All lanes reopened shortly after 10:30 a.m.
Several hours before the Monrovia crash, a wrong-way driver collided head-on with another vehicle in a deadly, separate crash on the 405 Freeway in West Los Angeles.
City News Service contributed to this report.