A search was underway late Thursday for a man who jumped into Lake Minnetonka’s Lower Lake to aid a struggling swimmer, then himself disappeared beneath the waves.

The Hennepin County Sheriff’s Water Patrol was called to the area about 6:30 p.m., the Sheriff’s Office said in a news release.

A woman who was among nine adults on board a pontoon boat jumped into the water and began to struggle. Two women and a man jumped in to try to help her. The three women made it back to the pontoon boat, but the man disappeared, the Sheriff’s Office said.

The area was being searched late Thursday with sonar.