VIDEO: A small plane crashed in southern Wisconsin Sunday, June 10 -- killing four people -- a 17-year-old girl, a 13-year-old boy and their mother and grandmother. The Green County sheriff said the crash happened just after noon on Sunday about a mile north of the Monroe Municipal Airport. The grandmother was the pilot. "It's a tragedy. The victims were en route here for a graduation celebration I believe was going on and to have this kind of news is just so disheartening," said Sheriff Mark Rohloff. "We knew seeing that much smoke it was something very serious," said Bryan and Nathan Holland, who headed to the scene and captured photos. "A lot of emergency vehicles. We could see two distinct areas where smoke was coming from."