WASHINGTON - The White House has directed the CIA to declassify parts of a Senate report criticizing harsh interrogations of suspected terrorists, but history shows that the agency is accomplished at preventing embarrassing or damaging disclosures.

In recent years, the CIA has wrestled with Congress, archivists, journalists, former employees and even an ex-director over which secrets could be revealed.

Most often, secrecy prevails.

The CIA holds the upper hand, using internal reviews of classified materials and a separate process to scan proposed books about intelligence practices to tightly guard what is known about agency activities and history.

"They're tightfisted by nature and the more they are pressed to disclose, the more they resist," said Steven Aftergood, who studies government secrecy at the Federation of American Scientists.

The CIA's experts have begun a review of the Senate Intelligence Committee's 400-page summary and findings on the agency's harsh interrogation techniques, according to government officials familiar with the process.

CIA spokesman Dean Boyd said the CIA, with help from other agencies, including the Pentagon and the Departments of Justice and State, is carrying out an "expeditious classification review" of the Senate materials. Boyd and others would not estimate when it would be completed.

The committee's leader, when referring the report to President Obama this month, asked that the White House take the lead in declassifying the summary. It criticizes the CIA for its heavy use of the simulated drowning technique called waterboarding and other abusive interrogation methods against al-Qaeda suspects held in secret, agency-run prisons overseas.

Sen. Dianne Feinstein (D., Calif.) and CIA Director John Brennan have clashed over the handling of agency documents.