Randy Wilcox, who runs the website DetroitFunk.com, said he used a claw hammer to slice the nylon straps and wire that held the sign up, the Detroit Free Press reported Wednesday.
"Like most reasonable people, I was shocked and disgusted at the use of a concentration camp sign in such a fashion," Wilcox said. "I can't imagine what the people who did it were thinking."
It was unclear who was behind the sign as Detroit residents debated whether it was an act of hate or intended as satire about the economy.
Attorney John Bologna who represents the plant owner called the sign "an abomination."
The Packard Motor Co. produced one of the most famous U.S. luxury cars of the early 20th century. Production of Packards stopped in 1958 and the plant fell into disrepair.