WASHINGTON (AP) — The Interior Department’s internal watchdog says Secretary Ryan Zinke has failed to keep complete records of his taxpayer-financed travel — hampering an investigation into his use of private charter flights for government business.

Deputy Inspector General Mary Kendall says her investigation “has been delayed by absent or incomplete documentation for several pertinent trips and a review process that failed to include proper documentation.”

Kendall said Interior employees have cooperated with her office, but said investigators have found that documentation and adherence to department policies are “deficient and without proper management oversight and accountability.”

Zinke has brushed off news reports that he took at least three private flights costing taxpayers a total of $20,000 since taking office in March, saying all his travel is ethical and went through proper due diligence.