"I am writing to request an Inspector General review of when and how individuals are provided tours of and access to Air Force One, and of which individuals have been provided tours of or access to the plane since January 20, 2017," Senators Elizabeth Warren, Tom Carper, Richard Blumenthal and Sheldon Whitehouse said in their letter to Pentagon Acting Inspector General Fine and Air Force Inspector General Lt. Gen. Stayce Harris.
The letter comes after US media reported in July that some members of Trump's Florida clubs appear to have been invited to join Air Force One Tours. At least two such tours appear to have taken place, according to the reports.
"Previous Presidents have provided friends and even campaign donors tours of Air Force One, but the reports that members of the President's private club may have received such tours are particularly troubling because their relationship to the President arises out of an ongoing business relationship with the President," the letter said. "If true, these reports could mean that the President or his family have used government property for private gain."
READ MORE: UK Tabloids Compare Putin and Trump's Presidential Planes Using Dubious Photos
The lawmakers said such a use of Air Force One could represent a violation of federal ethics rules.
The United States of Government Ethics regulations outline the basic obligations of public service and require that federal officials "shall not use public office for private gain,” they explained. Moreover, federal ethics law prohibits the use of government property, including Air Force One, "for other than authorized purposes".
READ MORE: Donald Trump Enraged As Melania's TV Set to CNN Aboard His Flight — Reports
While ‘Air Force One’ can describe any Air Force aircraft that carries the US President, the term is now used with regard to certain planes equipped to transport the commander-in-chief, according to the White House.