Edwin Donovan, spokesman for the Secret Service, told AFP: "We're aware of the incident, and we're going to take appropriate investigative steps.”
Gwendolyn Crump, spokesman for the Washington police, said: "We received an allegation that inappropriate comments were made. We are currently investigating the nature of those comments.”
The first lady's office did not immediately comment on the report.
In 2009 an American woman was arrested after calling the Secret Service and threatening to kill the Mrs Obama.
Barack Obama, the first African American president, has been the target of numerous assassination plots and threats since his campaign for the Democratic nomination in 2007.
Source: Agencies
