In a letter obtained by Buzzfeed on Thursday, Senators Elizabeth Warren (D-MA), Tom Carper (D-DE), Richard Blumenthal (D-CT), and Sheldon Whitehouse (D-RI), request details from the Inspector General on who has received Air Force One tours since January 2017, and whether their tours violate any regulations.

“If true, these reports could mean that the President or his family have used government property for private gain — giving tours of Air Force One as a perk for club membership — or that some citizens have potentially been able to buy access to Air Force One via their personal payments to the President’s private club,” reads the letter.

Reports of the president’s private tours first came out in early July. While it’s not uncommon for presidents to give Air Force One tours to friends or even campaign donors, this situation is unique due to the business relationship between Trump and the Mar-a-Lago members who reportedly received tours.

“This use of Air Force One would not only raise questions about readiness, security, and taxpayer cost, but it also violates the basic principle against using public office for private gain, and could constitute a violation of federal ethics rules,” wrote the Democratic senators.

In their call for an investigation, the senators asked for a complete list of those who received tours, as well as information on how they were vetted for security. They inquired about whether rules or regulations were broken in giving these tours, and asked for a copy of Air Force One’s policies on “authorized purposes” for usage.

They also asked if the individuals who received tours had business relationships with Trump, and “if so, whether the existence of this relationship was a factor in giving them access to the plane.”