It's not clear what led to the crash.
Ten people were injured and four more are missing after two boats collided head-on Saturday night on the Colorado River.
Helicopters and divers have been deployed in an effort to find the three missing women and one man, Mohave County Sheriff Doug Schuster said. Other officials were combing the shorelines, searching for survivors along the river that divides California and Arizona.
"These efforts will continue until all of those who are missing and unaccounted for have been located," said Schuster.
All together, 16 people were aboard the two boats. Some of the passengers were thrown from the boats, then carried miles down the river before they were found.
It's not clear what led to the crash, though authorities believe the darkness contributed. None of the passengers on either boat were wearing life jackets, officials said.
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