The incident occurred on September 16 in Atlanta while Obama was visiting the US Centers for Disease Control and Prevention to discuss the response to the Ebola virus, Washington Post reported today.
"A security contractor with a gun and three convictions for assault and battery was allowed on an elevator with President Obama during a September 16 trip to Atlanta, violating Secret Service protocols," the paper said.
He did not comply when Secret Service agents asked him to stop, the report said.
The agents questioned him, and used a database that showed he had a criminal record - three convictions for assault and battery.
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A supervisor from the security firm fired the contractor on the spot. The man then agreed to turn over his gun, which he should not have had in Obama's presence.
The report in the Post could be another embarrassment for the Secret Service after its director Julia Pierson took responsibility before a hostile House of Representatives oversight committee hearing for an "unacceptable" security breach at the presidential residence.
That incident came after a previous security scare in 2011, when a lone gunman fired shots at the White House.
The following year three Secret Service agents were sent home from a presidential trip to Amsterdam after they were caught drinking.
"You have a convicted felon within arm's reach of the president and they never did a background check," Jason Chaffetz, who heads a subcommittee investigating the Secret Service, said while reacting to the latest incident.