Sanders asked Mattis to address the following three issues:
– What Sanders considers the excessive compensation of defense contractors.
– What the senator considers widespread misconduct and fraud in the defense contracting industry.
– Massive cost overruns in the Pentagon's acquisition budget.
"Corporate interests should never take precedence over the interests of taxpayers or our national security. But paying exorbitant salaries to defense contractor CEOs makes that outcome more likely, and that is simply unacceptable," Sanders wrote.
The CEOs of the top five U.S. defense contractors earned a combined $96 million in compensation last year.
The Defense Department declined to comment on the letter. "Any conversations between the Secretary and other officials are private matters, and we do not discuss the nature or content," a Pentagon spokesman told CNBC.
The letter comes one week after Sanders grilled Pentagon comptroller David Norquist over the multimillion-dollar salaries defense company CEOs earn.
In the letter, Sanders requested "a list of recommendations on reducing excessive defense contractor compensation, and what steps, if any, DoD is taking to address this issue."
Finally, Sanders called on Mattis to hold defense companies accountable for the approximately $484 billion in cost overruns and operational delays.
"Many DoD programs fall short of cost, schedule and performance expectations, meaning DoD pays more than anticipated, can buy less than expected, and, in some cases, delivers less capability to the warfighter," Sanders wrote, citing the Government Accountability Office.
Sanders laid out his concerns about defense contractors' executive pay in a Senate Budget Committee hearing on the Defense Department's business operations.