According to Heavy website:
1. Anthony Ferrill, Who Worked at the Brewery for More Than 20 Years, Killed Himself After Fatally Shooting 5 Co-Workers, Police Say.
So either Fox is wrong or Heavy is wrong with regards to the number of years he was employed there.
The prominent one, including from local sources, appears to be that he was an electrician for 20 years, 17 of them at Molson Coors.
Milwaukee Journal Sentinel
"Anthony N. Ferrill, 51, had worked as an electrician for more than 20 years, about 17 of them at Molson Coors, according to multiple sources and online employment records."
https://www.jsonline.com/story/news/2020/02/27/milwaukee-miller-coors-shooting-anthony-ferrill-identified-molsoncoors-gunman-shooter/4891164002/
Perhaps “electrician” for 17, but maybe worked there in another capacity, part-time pre-trade?