By Aruna Viswanatha and Mark Hosenball

WASHINGTON (Reuters) - U.S. prosecutors have declined to pursue criminal investigations into a heated dispute between Senate investigators and the Central Intelligence Agency over documents related to its use of "enhanced interrogation techniques," the U.S. Justice Department said on Thursday.

The CIA's inspector general and its general counsel had both asked the Justice Department to get involved amid accusations that Senate Intelligence Committee staffers inappropriately accessed the CIA documents, and that agency operatives improperly monitored the Senate investigators.

Prosecutors delivered letters to both CIA offices declining to pursue any charges on Wednesday, the Justice Department said.

"The department carefully reviewed the matters referred to us and did not find sufficient evidence to warrant a criminal investigation," DOJ spokesman Peter Carr said in a statement.

The CIA declined comment. Senator Dianne Feinstein, who chairs the Senate Select Committee on Intelligence, said she was pleased the Justice Department had decided not to open an investigation into committee staff.

"I believe this is the right decision and will allow the committee to focus on the upcoming release of its report on the CIA detention and interrogation program," Feinstein said.

The review began after members of Congress complained that CIA officers had improperly accessed the work of committee staffers.

The review was also expected to look at allegations that Senate investigators inappropriately got access to what the agency considered to be ultra-sensitive and privileged documents related to its internal assessments of the program the CIA used to grab, hold and question militants after the Sept. 11, 2001, attacks.

After examining an estimated six million pages of CIA documents relating to the program, the Democratic majority on the Senate committee drafted a 6,000-page report that is still highly classified.

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Feinstein has said, however, that the CIA recently finished its declassification review of the report, which is now being reviewed at the White House.

Reuters earlier reported that the report was highly critical of some CIA activities, notably the use of "enhanced interrogation techniques," such as simulated drowning which human rights advocates and numerous U.S. politicians denounced as torture.

The dispute between the agency and Senate investigators arose after committee officials began asking the CIA questions about what the agency considered to have been confidential documents to which Senate staffers should not have had access.

After receiving Senate inquiries, the CIA looked at access logs and discovered that the staffers had accessed what the agency considered material covered by legal or other official privileges, and apparently made copies for themselves.

News reports then suggested that the CIA may have eavesdropped on congressional computers as part of its effort to find out what the Senate investigators were up to.

(Reporting by Aruna Viswanatha and Mark Hosenball, editing by G Crosse and; Doina Chiacu)