Updated

The late-night boat crash that killed four people along the Mississippi River over the weekend involved two small fishing boats that were shuttling partiers to shore in southeast Iowa, authorities said Monday.

Those involved had been partying on an island sandbar on a section of the river near the small town of Burlington, Des Moines County Sheriff Mike Johnstone said. The flat-bottomed aluminum boats crashed nearly head-on while shuttling people to a dock downriver around 2 a.m. Saturday. At least one boat didn't have lights.

A newer boat had dropped people off and was returning for more when it hit an older boat with 11 people on board. The front of the older boat was torn off, and all of its occupants were thrown into the water, Johnstone said.

The newer boat, which sustained minor damage, was able to rescue all but four people whose bodies were pulled from the water Sunday within 100 yards of the crash site. The victims were identified as Jacob Boyd, 22; Matthew House, 21; Blake Eakins, 21; and Caitlyn Atchley, 20. All were from Burlington.

"They were all just local friends that hang out on the river," Johnstone said. "We're a river town and we all go out on the weekends, and different groups hang out in different areas."

The eight survivors didn't suffer life-threatening injuries, Johnstone said.

He said alcohol was served at the party but it's too early to tell whether it was a factor in the collision. The operators of both boats have taken blood-alcohol tests, though the results haven't been released. It is illegal in Iowa to operate a boat with a blood-alcohol level above 0.08.

Iowa Department of Natural Resources Officer Paul Kay told radio station KBUR that both boats had life jackets, but it appears they weren't being used. Iowa law requires a boat to carry life jackets for each passenger, but it only requires them to be worn for passengers age 12 and under.

Authorities planned to continue interviews this week.

There was no moonlight at the time of the crash, which occurred about four miles north of the U.S. Highway 34 bridge that crosses the Mississippi River at Burlington. One boat had no lights and it's unclear if the other did, Johnstone said.

The boat carrying 11 people was built in 1977, and it wasn't immediately clear if it had a Coast Guard rating that dictated the total weight it was allowed to carry. Still, the operator of the boat is responsible for ensuring it is not overloaded, Kay said.

Johnstone said there are laws against operating a boat in an unsafe manner that can be enforced when a boat is loaded beyond safe capacity.

"Every boat has a limit to what it can carry," he told The Associated Press in an interview. "I can tell you from my personal experience 11 was too many for that boat."

One boat was 18 feet long the other was 20 feet. They were both jon boats, a term used for boats with flat bottoms, usually made of aluminum and frequently used for fishing.

Johnstone and Kay stressed boating safety as the Memorial Day holiday approaches and water recreation increases.

Iowa Gov. Terry Branstad opened his weekly news conference Monday by discussing the accident, echoing the other officials' warnings.

"I want to remind Iowans that it's important that we put safety first whenever navigating our waterways and our highways," he said. "We need to be alert, we need to be prepared and if we're in a boat we need to be wearing lifejackets."