"I'm seeing confederate flags on the walls, the dining room table and even the garage," he said.
"I'm thinking to myself as a joke, 'I'm walking to the imperial wizard of the Ku Klux Klan's house right now.' So, to my surprise, I walk into the bedroom (and) there's an application for the Ku Klux Klan in a frame on the wall. And this is the home of an officer of the Muskegon Police Department."
Mathis didn't name the officer. City Manager Frank Peterson confirmed to MLive.com that Anderson, who is white, owns the home the Mathis family toured.
"I said, 'I want to get out (of) here right now,'" Mathis told WOOD-TV. "To know that I was walking around on property associated with some type of racism, some type of hate, when I got outside I felt like I needed to be dipped in sanitizer."
City officials said in a statement Thursday that an internal investigation was launched after officials were made aware of Mathis' social media post.
"The officer was immediately placed on administrative leave, pending a thorough investigation," officials said.
Mathis told WZZM-TV the investigation was a "step in the right direction."
"If his intentions aren't good, he does not need to be a police officer," Robert Mathis' wife, Reyna Mathis, told WZZM-TV. "We, and all minorities are in harm's way if that's the case."
Anderson has been a member of the Muskegon Police Department for more than 20 years, Peterson told MLive. Anderson told the news site he was instructed not to talk about the situation as the internal investigation continues. His wife, Rachael Anderson, told WOOD-TV that he is not a member of the Ku Klux Klan.
Anderson has faced scrutiny before. He shot and killed a 23-year-old black man, identified as Julius Johnson, after the man ran from a traffic stop in 2009, WZZM-TV reported. In court, Anderson said Johnson had gotten ahold of his radio and baton, and that he used both to beat him during the confrontation, the news station reported. He said he feared for his life when he fired the shot that killed Johnson, an investigation report by then-Muskegon County Prosecutor Tony Tague said, according to MLive.
The NAACP called for a federal investigation into the shooting, but none was ever launched and Anderson was later cleared in the shooting, WOOD-TV reported.
Johnson's sister was convicted of lying to police after she told investigators she heard her brother beg for his life, according to WZZM-TV. Prosecutors said she was not close enough to the scene to have heard such a thing, WOOD-TV reported, and she was sentenced to three months in jail.