Iran is providing deadly bombs to Shiites and Bush will release some of the intelligence to support that assertion today:

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.
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.
[…]
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.
[…]
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.

Read the rest here.