Update | After shift in tone, Senator Dianne Feinstein is hailed for attack on CIA

Secret reports. Vanishing documents. Whispers of crime, intimidation and cover-up.

A quarrel between the CIA and the US Senate that's been rumbling beneath the surface for years burst into full view on Tuesday when Senator Dianne Feinstein laid it all out in eye-popping detail on the Senate floor.

Feinstein's outspoken criticism of "CIA interference" in a congressional investigation is in sharp contrast to her defence of an intelligence-gathering community that some say tramples on civil liberties.

Feinstein, who chairs the Senate intelligence committee, has fretted that the public was losing confidence "in the dedicated men and women of our intelligence community" because of a string of disclosures that she said often lacked important context. In particular, she has defended the collection by the National Security Agency (NSA) of massive amounts of phone records, revealed in detail by former NSA contractor Edward Snowden.

Now the California Democrat has turned critical, claiming that Congress was the target of intelligence-gathering. In a Senate floor speech on Tuesday she accused the CIA of criminal activity in searching a computer network set up so lawmakers could review secret documents provided for an investigation into the use of harsh interrogation techniques.

The American Civil Liberties Union called her speech a "forceful, necessary and historic defence of the constitutional principle of separation of powers".

"After so many years of Congress being unable or unwilling to assert its authority over the CIA, Senator Feinstein today began to reclaim the authority of Congress as a check on the executive branch," said Christopher Alders, senior legislative counsel for the union.