The most deadly improvised explosive devices being used against U.S. soldiers in Iraq continue to come from Iran, and Iran continues to provide more tactical training, according to explosive experts working with the U.S. military.
The Iranian-made devices are known as EFPs, or Explosively Formed Projectiles. When exploded, the copper disc center becomes a molten liquid bullet that can penetrate the thickest armor the United States has.
Earlier this month, CIA Director General Michael Hayden told the Senate Select Intelligence Committee that the EFPs from Iran cause more casualties on an incident-for-incident basis than any other type of improvised explosive device.
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"EFPs are coming from Iran. They are being used against our forces," said Hayden. "They are capable of defeating some of our heaviest armor."
ABC News first reported on the use of EFPs last March when U.S. officials told ABC News that they discovered the link to Iran via tell-tale manufacturing signatures -- certain types of machine-shop welds and material indicating they are built by the same bomb factory.
"I think the evidence is strong that the Iranian government is making these IEDs, and the Iranian government is sending them across the border and they are killing U.S. troops once they get there," says Richard Clarke, former White House counterterrorism chief and an ABC News consultant.
U.S. intelligence officials say Iran is using the bombs as a way to drive up U.S. casualties in Iraq without provoking a direct confrontation, but a looming question remains. According to CIA Director Hayden and others, most of the EFPs are provided to the Shia militias, while it is the Sunnis who are responsible for many more U.S. deaths. Officials are now asking, could Iran be arming both sides of the sectarian violence?
Either way, Clarke says the evidence is clear that the Iranians know they are causing damage to the U.S.
"I think it's very hard to escape the conclusion that, in all probability, the Iranian government is knowingly killing U.S. troops," said Clarke.
Iran has denied supplying weapons to fighters in Iraq saying the country only has political and religious ties to Iraq's Shiites.
Today U.S. officials said the presentation of evidence against Iran by U.S. military officials in Iraq would be delayed until next week or possible the week after due to concerns over revealing sensitive intelligence.
Some of that evidence, according to U.S. military officials, includes Iranian-made weapons seized in Iraq with manufacturing dates as recent as 2006, suggesting brand-new weapons are going directly from Iranian factories to militias fighting U.S. troops in Iraq. The U.S. has also seized documents that allegedly prove Iranian intelligence is arming and training Shia militias.
The delay comes after a very strong warning to Iran from the U.S. Secretary of Defense late last week.
"We're not simply going to stand by and let people bring sophisticated IEDs into the country that can disable an Abrams tank, and give them a free pass," said Gates.
ABC News' Jonathan Karl contributed to this report.