A search resumed Sunday for four people missing after two boats collided head-on and sank one day earlier in the Colorado River near San Bernardino County’s Moabi Regional Park, along the California-Arizona border, officials said.

With a total of 16 people on the two boats, the crash injured nine of them and another two were left in critical condition and airlifted from the scene for treatment, Mohave County Sheriff Doug Schuster said at a Sunday afternoon news conference.

Shoreline searches and diving operations were being done to find the missing people, a male and three females, he said. As those efforts continue, it’s still unclear whether they were victims of drowning or survived, according to Schuster.

“Technically, it’s a dive mission looking for potential drowning victims,” he said of the underwater searches. “But I don’t want to give the interpretation that there isn’t a remote possibility that they made it to shore.”

Shortly after 8 p.m. on Saturday, the Mohave County Sheriff’s Office received several 911 calls regarding the crash in the river between Pirate Cove and the Topock Marina, north of Lake Havasu, the agency stated on Facebook.

Callers told dispatchers that multiple people were in the water, several individuals were missing and there were multiple injuries as well.

Units from the Sheriff’s Office’s Division of Boating Safety responded to the scene. They determined that a Hallett boat, occupied by 10 people, and a Sleek Craft, with six people on board, collided head-on, authorities said.

“During the collision, all subjects from both boats were ejected into the water, and both boats sank,” the statement read.

The four missing people had been riding on the Hallett boat, officials said. Their ages have not been released, but authorities said they were all from outside the area.

Passing boaters pulled several other people from the water. Deputies and a paramedic located one person, only identified as a female, in a good Samaritan’s boat. She was flown to a Las Vegas-area hospital in critical condition, according to authorities. Details about the other victim in critical condition were not released by authorities.

Meanwhile, nine other people were transported to area hospitals by ambulance, the Sheriff’s Office stated.

People involved in the crash were not wearing life jackets — a practice that’s not legally required by recommended by authorities, Schuster said.

He said an area of the river about 2 miles in circumference is being searched, and diving efforts will stop at nightfall and resume the next morning if there’s no progress on Sunday.

The San Bernardino County Sheriff’s Department was assisting the Mohave County Sheriff’s Office in the search operation. Before an afternoon news conference held by Mohave County authorities, the San Bernardino County Sheriff’s Department said several hours earlier that there was a total of 13 patients, including one person who suffered life-threatening injuries and some who were treated and released at the scene with minor injuries.

The cause of the crash is under investigation.