A U.S. Senate inquiry on the CIA's treatment of terrorism suspects will probe the usefulness of intelligence gained through harsh interrogations and whether the agency complied with official legal guidance.

Senate Intelligence Committee Chairman Dianne Feinstein, a Democrat, and Republican Vice Chairman Kit Bond announced on Thursday the scope of the inquiry into the CIA's controversial detention and interrogation program. They said it would take about a year.

In the program launched after the Sept. 11 attacks, suspects were subjected techniques including simulated drowning, or "waterboarding," that were denounced as torture by human rights advocates, some U.S. officials, and international governments.

"The purpose is to review the program and to shape detention and interrogation policies in the future," the two lawmakers said in a statement.

The inquiry, to begin "soon," will be conducted in secret, a congressional aide said, and it is unclear whether findings will be made public. The committee will have the power to subpoena witnesses but "this is not a witch hunt," the aide said.

Human rights advocates and some leading Democratic senators have urged an open "truth commission" investigation into Bush-era the antiterrorism policies including the CIA's program.

They say it is important to air excesses in the policies to prevent a recurrence.

Feinstein and Bond said their investigators would conduct an extensive review of documents and interview relevant participants. The inquiry would study:

-- The intelligence gained through harsh and regular interrogation techniques.

-- How the program was created and operated.

-- How the CIA assessed whether detainees had relevant information.

-- Whether the CIA accurately described the program to Congress and the Justice Department.

-- Whether the agency complied with Justice Department legal guidance and its own internal policies.

CIA Director Leon Panetta said in a message to employees the agency would cooperate with the probe. He said he had discussed it with Feinstein and Bond.

"They assure me that this review is a way for the committee to assess lessons learned from the program while our government devises a new interrogation policy."

"What I will not support is an inquiry designed to punish those who acted in accord with guidance from the Department of Justice. Senators Feinstein and Bond have made it clear to me that this is not the goal of this review," he said in the letter, a copy of which was obtained by Reuters.

Former CIA Director Michael Hayden has insisted that agency employees acted with precise legal guidance, and that the program gained valuable intelligence that saved lives.

The CIA has said fewer than 100 suspects were held in the secret program, and about one-third of them were subjected to harsh techniques. It said three suspects experienced "waterboarding."