WASHINGTON - Defense Secretary Robert Gates wants to know why his military uses private contractors like Moyock, N.C.-based Blackwater Worldwide for combat and security training, and how widespread the practice is.

Gates is asking for answers from the Pentagon's top military officer, Joint Chiefs of Staff Chairman Adm. Mike Mullen.

"In my mind, the fundamental question that remains unanswered is this: Why have we come to rely on private contractors to provide combat or combat-related security training for our forces?" Gates wrote in a memo to Mullen that was released Monday to The Associated Press by the office of Sen. Jim Webb, D-Va.

"Further, are we comfortable with this practice, and do we fully understand the implications in terms of quality, responsiveness and sustainability?"

Gates' memo came after Webb raised concerns about the role of private contractors, and specifically Blackwater, which opened a new counterterrorism training center in San Diego last month over the opposition of city officials.

Webb told Gates in a letter that there's a "need for more rigorous, senior-level oversight" of contracts that go to private security contractors.

Such contracts must exceed $78.5 million before getting reviewed by service secretaries, Webb said. So the Navy's fixed-price contract for Blackwater's Lodge and Training Center in Moyock, which had a ceiling price of $63.8 million -- of which $53.2 million has been obligated -- would have escaped such scrutiny, Webb wrote. He said he was told the contract was initially valued at $35.9 million.

"Clearly, the size of these contracts and the relatively low level at which such contracts can now be approved should give all of us pause," Webb wrote.

In his memo to Mullen, Gates sought details on what percentage of military training is conducted by private contractors, how much it has cost over the past decade, and whether "appropriate red lines" have been established to determine what types of security training can be contracted out and what can't.

Gates told Webb he expects a response from Mullen next week.