WASHINGTON, Sept. 25  The Democratic chairman of a House committee complained Tuesday that the State Department was blocking his panel’s efforts to investigate the private security firm Blackwater USA and its operations in Iraq.

The department described the situation as a “misunderstanding.”

In a letter to Secretary of State Condoleezza Rice, Rep. Henry A. Waxman of California, the chairman of the House Oversight Committee, wrote that the State Department had prevented Blackwater from cooperating.

“Blackwater has informed the committee that a State Department official directed Blackwater not to provide documents relevant to the committee’s investigation into the company’s activities in Iraq without the prior written approval of the State Department,” Mr. Waxman’s letter stated. The letter was made available to the news media on Tuesday.

In response, a State Department statement late Tuesday said: “There seems to be some misunderstanding with regard to this matter. All information requested by the committee has been or is in the process of being provided.”