The man who police said opened fire at a Molson Coors plant in Milwaukee this week — killing five co-workers and himself — was a gun-obsessed electrician who built his own firearms, according to a report.

Anthony Ferrill — who authorities said unleashed one of the worst mass shootings in Wisconsin history on Wednesday — built guns with mail-order parts as a hobby, his next-door neighbor Erna Roenspies told journaltimes.com.

The 51-year-old disgruntled Coors employee had worked in the plant for 15 years before slipping off a ladder and hurting his shoulder, forcing him to miss work, said Roenspies, 82.

After the fall, he told her that “spies” from the beer company had been sent to the neighborhood to watch his movements and make sure he wasn’t faking the injury. He once pointed to a “spy” in a car and griped that he “irritated” him, she said.

But she added that Ferrill was always a “gentleman” with her.

On Wednesday, Ferrill reportedly was fired from the firm before returning to the 82-acre beer facility to carry out the bloody massacre with a silenced gun while wearing a stolen name tag.

All five victims were Coors employees who worked as electricians and machinists, according to police: Jesus Valle Jr., 33, of Milwaukee; Gennady Levshetz, 61, of Mequon; Trevor Wetselaar, 33, of Milwaukee; Dana Walk, 57, of Delafield; and Dale Hudson, 60, of Waukesha.

The Coors plant includes corporate offices and brewing facilities, and is one of the country’s biggest breweries.