President Donald Trump called for an investigation into alleged voter fraud, including whether citizens are registered to vote in more than one state, but least one member of his family, one of his senior aides, and a Cabinet nominee have been found to be registered in at least two states.

"I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and even, those registered to vote who are dead (and many for a long time)," Trump tweeted Wednesday. "Depending on results, we will strengthen up voting procedure."

Trump's nominee to be the secretary of the Treasury, former Goldman Sachs partner Steven Mnuchin, is registered to vote in both New York and California, according to a review of documents by CNN .

Steve Bannon, a senior adviser to the president and the former executive director of Breitbart News, a media outlet the Southern Poverty Law Center has described as "embracing ideas on the extremist fringe of the conservative right," registered to vote in New York before the election last year at the same time as he was registered to vote in Florida. Bannon is still registered to vote in Florida.

And Tiffany Trump, the president's younger daughter with his second wife, Marla Maples, is registered to vote in both Pennsylvania and New York.

Although it is not illegal to be registered to vote in multiple places – unlike voting twice in the same election, which is against the law – proponents of theories of voter fraud point to multiple registrations as proof that illegal voting takes place.

Mnuchin is registered at a Park Avenue home in Manhattan, where he last voted in 2008. Records in California show he is registered at an address in Bel Air, and he voted in

California's Republican primary last year. A Los Angeles county election official told CNN that files have not been updated for November's general election.

During the election, it was revealed by The Guardian that Bannon was registered in Florida at a vacant house in Miami-Dade County . The owner of the property said at the time he had never lived at the address where he was registered, a possible violation of Florida election law that prohibits "willful misrepresentation" of residency and bars registering some place that is not a primary residence. Bannon changed his Florida registration to the home of a friend after The Guardian's report in August; it's not clear if he ever lived at that home either.