(CNN) The Interior Department's internal watchdog has chastised agency officials for poor record-keeping as she investigates Secretary Ryan Zinke's travel expenses.

"Our investigation has been delayed by absent or incomplete documentation for several pertinent trips and a review process that failed to include proper documentation and accountability," Inspector General Mary Kendall wrote in a Wednesday memorandum.

Much of the required paperwork from the fiscal year that ended more than a month ago has "yet to be completed and processed," she said, and the records are unclear about instances when Zinke's wife accompanied him on business trips.

Her memorandum, known as a "management advisory," asked the department to develop stronger record-keeping procedures and provide the full documentation.

Deputy Interior Secretary David Bernhardt responded that he believes the department has provided the requested documents and that Zinke "inherited an organizational and operational mess from the previous administration."

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