Hillary Clinton's top aide at the State Department spent at least some of her time on the federal payroll helping the nonprofit Clinton Foundation, according to a Thursday report.

Cheryl Mills, a longtime Clinton aide whose relationship with the family reaches back decades, hopped on an Amtrak train from Washington, D.C., to New York on June 19, 2012, an unnamed source told CNN, in order to interview two high-level business executives under consideration to lead the foundation.

The purpose of the trip has been shrouded in mystery to date, despite questions from Congress. A January 2016 letter on the topic sent to Secretary of State John Kerry by Iowa Sen. Chuck Grassley, the Republican chairman of the Senate Judiciary Chairman, went unanswered.

Mills maintains that her time spent working for the foundation constituted volunteer work. Federal statutes prohibit employees from working for private organizations while on the federal payroll, considered a misuse of public funds.

Asked about the trip, the State Department said in a statement that it should be considered ethical so long as Mills maintains it was purely volunteer work. "Federal employees are permitted to engage in outside personal activities, within the scope of the federal ethics rules," the department said. "All federal employees are subject to federal ethics laws and regulations, including rules pertaining to conflicts of interest."

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Clinton campaign spokesman Brian Fallon echoed the State Department. "Cheryl volunteered her personal time to a charitable organization, as she has to other charities," Fallon said. "Cheryl paid for her travel to New York City personally, and it was crystal clear to all involved that this had nothing to do with her official duties. The idea that this poses a conflict of interest is absurd."

The revelation follows a report earlier Thursday that the Justice Department declined to open a public corruption probe into the foundation, despite a recommendation to that effect from three Justice Department field offices. Emails from Clinton's time at the State Department released earlier this week as part of a Freedom of Information Act request appeared to show officials at the foundation interacting improperly with officials at State, though Clinton maintains all of the activity was above board.