(CNN) -- Boats with side-scanning sonar will be used in the search for four men believed to have drowned in a southeastern Idaho reservoir, a sheriff said Monday.
Power County, Idaho, Sheriff Jim Jeffries said he hopes to have the boats in the American Falls Reservoir early Monday afternoon. The boats include one from Elmore County in western Idaho and a Parks and Recreation Department boat from the state capital, Boise. In addition, Jeffries said he has sent a deputy to Salt Lake City, Utah, to purchase a third such boat.
The search began Sunday for the four boaters after authorities received a 911 call from a child on their boat, Jeffries said. The four men were "horsing around," he said. One man was standing up in the front of the boat and another man pushed him into the water as a joke, not realizing he could not swim, he said.
The man was "immediately in distress," Jeffries said, and the other men, one at a time, went into the 45-foot-deep water to try to help. The fourth man in the water grabbed a life jacket, but did not put it on, he said.
There were five children on the boat, ranging in age from 9 to 2, Jeffries said. One of the children grabbed a cell phone on the boat and called 911. The sheriff said he thought it was the 9-year-old but wasn't sure. Marine patrol deputies responded and took the children to safety, he said.
Authorities have a GPS coordinate on the boat from the county's E-911 mapping system, he said, and divers have set up a buoy line in the search. But "the diving is very difficult," he said. "... Visibility is poor. You can only see about to the end of your arm." In addition, the lines keep getting snagged on the reservoir's bottom, he said.
The men's names were not released because the families have not authorized the sheriff's office to do so, Jeffries said. "It's a little bit subjective of me to say, but I think if they release the names, it's like they've given up all hope."
He said the families were still attempting to get in touch with relatives and are "having a very difficult time processing all of this. They're in an extreme state of grief." Authorities are respecting their privacy, he said, and have set up a place for them on a hill overlooking the reservoir. Jeffries said a lieutenant was assigned to stay with the relatives.
The reservoir is about 180 miles east-southeast of Boise.
CNN Radio's Matt Cherry and CNN's Greg Morrison contributed to this report.