A white police officer in western Michigan has been placed on administrative leave after a prospective home buyer said he saw a framed Ku Klux Klan application and multiple Confederate flags in his house.

The Muskegon Police Department announced Aug. 8 it had opened an internal investigation after a social media post was "brought to its attention" accusing the officer, Charles Anderson, of "being in possession of certain items associated with a white supremacy group."

The veteran officer was immediately placed on administrative leave, according to the department.

The man behind the post, Robert Mathis, who is black, has subsequently received death threats.

Mathis posted a picture of the KKK document on Facebook on Aug. 7 after touring Anderson’s home with his wife, Reyna, their two children and a realtor.

The Ku Klux Klan document Robert Mathis says was on display at Muskegon Police Officer Charles Anderson's house. Robert Mathis / via Facebook

Reyna and Robert Mathis said they saw the application and several Confederate flags inside the house that is for sale in Holton Township, about 20 miles northeast of Muskegon. The couple believed they were in the home of a police officer because they also saw a police jacket and a photo of an officer in uniform.

"My emotions were all over the place. I felt anger, sadness and shame," Reyna Mathis, 42, who is Hispanic, told NBC News on Tuesday. "Our realtor, who is white, even cried. She just kept apologizing."

"It was just sitting there on that wall as if it was a prized possession of some sort," she said.

Her husband, who spotted the document first, was so upset that he fled the residence, she said.

Mathis, 52, a U.S. Army veteran, said he was exposed to some "racial things" in the military but that he dealt with it.

"As an adult, dealing with this stuff with my family, it just slapped me in my face so hard," he said.

Mathis said he tried ignoring the various Confederate flags on display as he toured the house.

"I kept telling myself, 'It’s not what you think' and 'You’re overthinking it,'" he said.

But the KKK application was too much to ignore, he said.

The couple, who live in Muskegon Township, said they debated last Wednesday night whether to share what they had witnessed. Mathis said he "prayed for a solution" and decided to post a lengthy statement on Facebook accompanied by a photo from Anderson's home.

"When I saw the Klan application and knew that he was an officer, it brought such a great concern to me because I knew he wouldn’t be effectively doing his job in the community," Mathis said.

Anderson did not immediately return a request for comment from NBC News.

The officer’s union, the Police Officers Labor Council, declined to discuss the matter Tuesday, stating it does not comment on pending issues.

The Muskegon County prosecutor’s office told MLive that it will wait for the results of the police department's inquiry before it decides whether to reevaluate a 2009 incident in which Anderson fatally shot a 23-year-old black man. Anderson was cleared in late 2009 of any wrongdoing in what was determined to be an act of self-defense.

Mathis, who was born and raised in Detroit, said he made his post public at the encouragement of his Facebook friends. He said he did not know or disclose the officer's identity in the post, but that other social media users identified him in the comment thread.

"If he’s been in the police force for over 20 years, I can’t even imagine the things he’s gotten away with and the things he’s said and done," Mathis said. "I just want people to know. You have to speak up."

What has hurt the most, Reyna Mathis said, was having to explain to their 12-year-old daughter why she and her husband were so upset. Mathis said he told his daughter the KKK is part of America's "dark history."

"We started explaining what the KKK stands for," Reyna Mathis said, adding that she believed the officer "was representing" the group by having one of its applications on display in his home.

On Sunday, Mathis was visited at his home by Muskegon Township police, who notified him and his family about a threat against their lives. The threat, which was posted on the Facebook page of a Fox affiliate in Michigan as a comment on a story about Anderson's suspension, read: “Those people looking at this house that took the picture, better open their eyes in the back of their head.”

The threat has since been deleted.

Muskegon Township Police Chief David Wypa said one of his officers was asked by a Muskegon officer to inform Mathis of the post. Wypa said Tuesday his department made no determination with the information it gave Mathis but was merely relaying it.

Despite the threats and backlash his family has faced, Mathis said he has no regrets about what he disclosed.

"The only reason I made this post is because if there is an injustice done to any minorities out there under the care of this officer, it should be scrutinized," he said.