U.S. military and intelligence officials tell ABC News that they have caught shipments of deadly new bombs at the Iran-Iraq border.

They are a very nasty piece of business, capable of penetrating U.S. troops' strongest armor.

What the United States says links them to Iran are tell-tale manufacturing signatures — certain types of machine-shop welds and material indicating they are built by the same bomb factory.

"The signature is the same because they are exactly the same in production," says explosives expert Kevin Barry. "So it's the same make and model."

U.S. officials say roadside bomb attacks against American forces in Iraq have become much more deadly as more and more of the Iran-designed and Iran-produced bombs have been smuggled in from the country since last October.

"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. "I think it's very hard to escape the conclusion that, in all probability, the Iranian government is knowingly killing U.S. troops."

'Very Lethal'

U.S. intelligence officials say Iran is using the bombs as a way to drive up U.S. casualties in Iraq but without provoking a direct confrontation.

John Negroponte, director of national intelligence, testified before the Senate Select Committee on Intelligence on Februrary 2, saying, "Tehran's intention to inflict pain on the United States and Iraq has been constrained by its caution to avoid giving Washington an excuse to attack it."...