"We received an allegation that inappropriate comments were made. We are currently investigating the nature of those comments," according to a statement from the D C metro police department.
The officer works as a motorcycle escort with the agency, but has since been moved to administrative duty, according to the Washington Post.
The motorman allegedly made the comments on Wednesday as several officers from the special operations division discussed threats against President Barack Obama. It was not immediately clear where the alleged conversation took place or exactly how many officers took part in the conversation.
During that conversation, the officials said, the officer allegedly said he would shoot the First Lady and then used his phone to retrieve a picture of the firearm he said he would use. It was not immediately clear what type of firearm was allegedly shown.
Police officials declined to identify the officer. Officials from the US attorney's office declined comment.
Police union chief Kristopher Baumann said he did not have details on the matter.
There was no indication of a legitimate danger to Michelle Obama. A Secret Service spokesman declined to provide further details, saying in an e-mail that the agency was aware of the incident and "will conduct appropriate follow-up".