U.S. military commanders in Iraq have shown members of Congress explosive devices that bear Iranian markings as evidence Tehran is supplying Iraqi militants with bombs, a senior U.S. government official said Saturday.

One of the lawmakers, independent Sen. Joe Lieberman of Connecticut, said he has seen some of the evidence, though he would not be specific. "I'm convinced from what I've seen that the Iranians are supplying and are giving assistance to the people in Iraq who are killing American soldiers," Lieberman said.

The senior official said military commanders in December showed lawmakers mortar rounds and other munitions and fragments that had Iranian serial numbers and markings.

The official, who requested anonymity because the evidence collected has not been made public, said U.S. generals also displayed improvised explosive devices that they said reflected Iranian style.

On Friday, Defense Secretary Robert Gates told reporters that serial numbers and other markings linked the Iranians to explosives used by insurgents in Iraq. His comments were among the Bush administration's first public assertions about evidence the military has collected.

The administration and military officials have said repeatedly that Iranians have been tied to terrorist bombings in Iraq. But U.S. officials have said little about evidence, including any documents and other items collected in recent raids in Iraq, to bolster such claims.

National security officials in Washington and Iraq have worked for weeks on a presentation intended to provide evidence for the administration's claims of what they say are Iran's meddlesome and deadly activities.

Officials say the materials — which in their classified form include slides and 2 inches of documents — provide evidence of Iran's role in supplying Iraqi militants with highly sophisticated and lethal improvised explosive devices and other weaponry.

Among the weapons is a roadside bomb known as an "explosively formed penetrator," which can pierce the armor of Abrams tanks with nearly molten-hot charges. One intelligence official said the U.S. is "fairly comfortable" it knows the source of the explosives.

The Iran dossier also lays out alleged Iranian efforts to train Iraqis in military techniques.

Government officials say there is some disagreement about how much to make public to support the administration's case. Intelligence officials worry the sources of their information could dry up.

Among the evidence the administration will present are weapons that were seized in U.S.-led raids on caches around Iraq, one military official in Washington said on condition of anonymity because he was not authorized to speak publicly.

Other evidence includes documents captured when U.S.-led forces raided an Iranian office Jan. 11 in Irbil in northern Iraq, the official said. Tehran said it was a government liaison office. The U.S. military said five Iranians detained in the raid were connected to an Iranian Revolutionary Guard faction that funds and arms insurgents in Iraq.

The assertions have been met with skepticism by some lawmakers still fuming over intelligence reports used by the administration to propel the country to war with Iraq in 2003. In fact, a report this week by the Pentagon's internal watchdog criticized prewar assertions by the Defense Department about Al Qaeda's connections to Iraq.

Gates told reporters in Seville, Spain, on Friday that markings on explosives provide "pretty good" evidence that Iranians are supplying either weapons or technology for Iraqi extremists.

"I think there's some serial numbers, there may be some markings on some of the projectile fragments that we found" that point to Iran, he said.

Gates' remarks left unclear how the U.S. knows the serial numbers are traceable to Iran and whether such weapons would have been sent to Iraq by the Iranian government or by private arms dealers.

Explosives have been a leading killer of U.S. forces in Iraq, where more than 3,000 U.S. troops have died in the nearly four-year-old war.

Last week, Gates said U.S. military officers in Baghdad had planned to brief reporters on what was known about Iranian involvement in Iraq but that he and other senior officials delayed the briefing to assure the information was accurate.

A White House spokeswoman, Dana Perino, said Friday that such information would come from U.S. officials in Iraq, though she did not say when.