WASHINGTON – Defense Secretary Robert Gates wants to know why his military uses private contractors for combat and security training, and how widespread the practice is.

He's 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 July 10 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 Worldwide, which opened a new counterterrorism training center in San Diego last month over the opposition of city officials.

Webb had been blocking Senate consideration of four civilian Defense Department nominees while waiting for answers. On Monday, Webb told Gates he was lifting his opposition to the nominees.

In a letter to Gates, Webb wrote that he found the “affirmative steps” outlined in Gates' memo reassuring.

Webb was secretary of the Navy under President Reagan and serves on the Armed Services Committee.

He told Gates that after getting briefed by the Navy last week he still believes there's a “need for more rigorous, senior-level oversight of the outsourcing contracts themselves.”

Such contracts must exceed $78.5 million before getting reviewed by service secretaries, Webb said he was told. So the Navy's firm fixed-price contract for Blackwater's Lodge and Training Center in North Carolina, which was initially valued at $35.9 million but later bumped up to $63.8 million, would have escaped such scrutiny, Webb wrote.

“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 also asked for 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 expected a response next week.

Also Monday, Blackwater Worldwide executives told The AP that the company plans to shift away from providing security and focus on training, aviation and logistics. The company has been under scrutiny since some of its contractors opened fire in a crowded Baghdad intersection last year, and 17 Iraqis were killed.