WASHINGTON, Feb. 9 — The most lethal weapon directed against American troops in Iraq is an explosive-packed cylinder that United States intelligence asserts is being supplied by Iran.

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A Deadly Weapon

The assertion of an Iranian role in supplying the device to Shiite militias reflects broad agreement among American intelligence agencies, although officials acknowledge that the picture is not entirely complete.

In interviews, civilian and military officials from a broad range of government agencies provided specific details to support what until now has been a more generally worded claim, in a new National Intelligence Estimate, that Iran is providing “lethal support” to Shiite militants in Iraq.

The focus of American concern is known as an “explosively formed penetrator,” a particularly deadly type of roadside bomb being used by Shiite groups in attacks on American troops in Iraq. Attacks using the device have doubled in the past year, and have prompted increasing concern among military officers. In the last three months of 2006, attacks using the weapons accounted for a significant portion of Americans killed and wounded in Iraq, though less than a quarter of the total, military officials say.

Because the weapon can be fired from roadsides and is favored by Shiite militias, it has become a serious threat in Baghdad. Only a small fraction of the roadside bombs used in Iraq are explosively formed penetrators. But the device produces more casualties per attack than other types of roadside bombs.

Any assertion of an Iranian contribution to attacks on Americans in Iraq is both politically and diplomatically volatile. The officials said they were willing to discuss the issue to respond to what they described as an increasingly worrisome threat to American forces in Iraq, and were not trying to lay the basis for an American attack on Iran.

The assessment was described in interviews over the past several weeks with American officials, including some whose agencies have previously been skeptical about the significance of Iran’s role in Iraq. Administration officials said they recognized that intelligence failures related to prewar American claims about Iraq’s weapons arsenal could make critics skeptical about the American claims.

The link that American intelligence has drawn to Iran is based on a number of factors, including an analysis of captured devices, examination of debris after attacks, and intelligence on training of Shiite militants in Iran and in Iraq by the Iranian Revolutionary Guard and by Hezbollah militants believed to be working at the behest of Tehran.

The Bush administration is expected to make public this weekend some of what intelligence agencies regard as an increasing body of evidence pointing to an Iranian link, including information gleaned from Iranians and Iraqis captured in recent American raids on an Iranian office in Erbil and another site in Baghdad.