MOABI REGIONAL PARK, Calif. (AP) — Four people are missing after two boats crashed and sunk on the Colorado River, ejecting more than a dozen people who were on board, authorities said Sunday.

A recreational boat carrying 10 people and another vessel with six people on board collided head-on Saturday night on a well-traveled stretch of the river that marks the border between California and Arizona, the Mohave County Sheriff’s Office said.

Both boats sank, and passing boaters pulled crash victims from the water. Nine people were injured, with two in critical condition, authorities said.

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The crash occurred between two popular marinas and near Moabi Regional Park, a well-traveled recreation area crowded with people enjoying the Labor Day weekend.

Three women and one man remained missing on Sunday, Mohave County Sheriff Doug Schuster said.

Helicopters have been deployed, and authorities have scoured the shoreline to try to find them. Divers have been sent out into the river’s waters, which can range from shallow to 30 feet (9 meters) deep.

“These efforts will continue until all of those who are missing and unaccounted for have been located,” Shuster told reporters on Sunday.

The cause of the crash is being investigated. None of the boaters were wearing life jackets, which aren’t required but are recommended by authorities, Shuster said.

On Saturday night, authorities had to temporarily halt the search because it was too dangerous to put divers into the dark waters. Efforts resumed after daybreak Sunday, and authorities closed off a stretch of the river where the search was taking place.

“We have four parties unaccounted for and presumed submerged,” said Eric Sherwin, spokesman for the San Bernardino County Fire District in California, which is assisting in the search.