On Friday afternoon, a sudden and severe storm system swept across Walworth County and the surrounding area, officials said.
The National Weather Service said peak wind gusts were between 90 and 100 mph.
The Lake Geneva Fire Department said they responded to 75 calls for service, including a dive rescue, two structure fires, two incidents involving trees on vehicles with entrapment and 37 calls involving downed wires or electrical hazards.
Police said hundreds of trees were downed, and buildings were damaged.
The storm created extremely hazardous boating conditions on Geneva Lake.
A privately owned 25-foot 2024 Nautique P25 vessel, which features a maximum capacity of 19 passengers, was navigating the lake during the storm, officials said.
A 47-year-old man with extensive boating experience was operating it, police said.
The others onboard included a 75-year-old man, 73-year-old woman, 49-year-old woman, 46-year-old man, 44-year-old woman, 12-year-old boy, 10-year-old girl, 7-year-old girl and 6-year-old boy.
Those on the boat are from Batavia, Wheaton and Fontana, Wisconsin.
As the weather worsened, the operator tried to seek safety, police said.
He was unable to and relied on his training and experience, turning the boat into the wind, according to police.
Large waves broke over the boat, causing it to take on water and list to the side.
It eventually rolled over and sank below the water.
A passing boat called 911, and first responders rescued seven people: six adults and one child.
Divers found the missing children, ages 10, 7 and 6, in the boat about 32 feet deep.
Officials said their preliminary cause of death is drowning.
The children's names have not been publicly released.
All children were wearing properly fitted life jackets, according to preliminary investigation.
The Wisconsin Department of Natural Resources and Geneva Lake Law Enforcement Agency are conducting a joint investigation.
Officials said other boats capsized, but no one else was killed.
Storm cleanup efforts continue Monday in parts of southern Wisconsin.
The need in Lake Geneva was so great, officials said more than 50 departments and agencies were brought in to assist with the storm damage.
Agencies from places such as Kenosha, Jefferson, Lake, McHenry and Boone counties assisted in the recovery efforts from the devastating storms last week.
"I kept watching that cloud and it was getting dark and darker," said Thomas Smoak, a visitor from Wheaton. "There were people crying, people very moved, hugging each other and we were praying."
Lake Geneva's mayor declared a disaster. A 211 hotline has been set up for people to report storm damage, and people in need of support can also reach out to the Red Cross.