London ? U.S. forces often failed to follow up on credible evidence that Iraqi forces mistreated, tortured and killed their captives in the battle against a violent insurgency, according to accounts contained in what was purportedly the largest leak of secret information in U.S. history.

The documents are among nearly 400,000 released Friday by the WikiLeaks website in defiance of Pentagon insistence that the action puts the lives of U.S. troops and their coalition partners at risk.

Although the documents appear to be authentic, their origin could not be independently confirmed, and WikiLeaks declined to offer any details about them. The Pentagon has previously declined to confirm the authenticity of WikiLeaks-released records, but it has employed more than 100 U.S. analysts to review what was previously released and has never indicated that any past WikiLeaks releases were inaccurate.

The 391,831 documents date from the start of 2004 to Jan. 1, 2010, mostly by low-ranking officers in the field. In terse, dry language, they catalog thousands of battles with insurgents and roadside bomb attacks, along with equipment failures and shootings by civilian contractors.

The documents describe a full gamut of a country at war: shootings at military checkpoints, contractors firing on Iraqis and savage acts committed on prisoners using boiling water, metal rods, electric shocks and rubber hoses. A group that counts casualties from the war said the files also document 15,000 previously unreported deaths.

The United States went to war in part to end the brutality of Saddam Hussein’s regime, but the WikiLeaks material depicts American officers caught in a complicated and chaotic conflict in which they often did little but report to their superiors when they found evidence that their Iraqi allies were committing their own abuses.

In some cases, the reports show the U.S. military intervening to protect detainees, but in many others officers did not act on what their troops described as clear evidence of abuse.

Allegations of torture and brutality by Shiite-dominated security forces — mostly against Sunni prisoners — were widely reported during the most violent years of the war when the rival Islamic sects turned on one another in Baghdad and other cities. The leaked documents provide a ground’s eye view of abuses as reported by U.S. military personnel to their superiors, and appear to corroborate much of the past reporting.

WikiLeaks said it provided unredacted versions of the reports weeks ahead of time to several news organizations, including the New York Times, Le Monde, The Guardian and Der Spiegel. It gave The Associated Press and other news organizations access to a searchable, redacted database hours before its general release Friday.

WikiLeaks was criticized for not redacting the names of informants in a July release of almost 77,000 documents from the Afghan conflict. This time, it appears to have removed the names of people, countries and groups from the searchable database.

WikiLeaks declined to make unredacted files available to the AP, saying journalists wanting such a copy would have to lodge a request with the organization, which would respond within a “couple of days.”

The group describes itself as a public service organization whose mission is to “protect whistle-blowers, journalists and activists who have sensitive materials to communicate to the public.”

WikiLeaks editor-in-chief Julian Assange did not respond to an e-mail from the AP seeking comment but told CNN that the documents show “compelling evidence of war crimes,” both by the U.S.-led coalition and the Iraqi government. Such comments from Assange have drawn controversy in the past.

Assange rejected claims that his work was endangering anyone.

The military has a continuing investigation into how the documents were leaked. An Army intelligence analyst stationed in Iraq, Spc. Bradley Manning, was arrested in connection with the leaking other classified material to WikiLeaks.

Pentagon press secretary Geoff Morrell called the release “shameful” and said it “could potentially undermine our nation’s security.”