If there is anywhere Iran could easily stir up trouble in Iraq, it would be in Diyala, a rugged province along the border between the two nations.
The combination of Sunni Arab militants believed to be affiliated with Al Qaeda and Shiite Muslim militiamen with ties to Iran has fueled waves of sectarian and political violence here. The province is bisected by long-traveled routes leading from Iran to Baghdad and Shiite holy cities farther south in Iraq.
But even here, evidence of Iranian involvement in Iraq's troubles is limited. U.S. troops have found mortars and antitank mines with Iranian markings dated 2006, said U.S. Army Col. David W. Sutherland, who oversees the province. But there has been little sign of more advanced weaponry crossing the border, and no Iranian agents have been found.
In his speech this month outlining the new U.S. strategy in Iraq, President Bush promised to "seek out and destroy" Iranian networks that he said were providing "advanced weaponry and training to our enemies." He is expected to strike a similar note in tonight's State of the Union speech.
For all the aggressive rhetoric, however, the Bush administration has provided scant evidence to support these claims. Nor have reporters traveling with U.S. troops seen extensive signs of Iranian involvement. During a recent sweep through a stronghold of Sunni insurgents here, a single Iranian machine gun turned up among dozens of arms caches U.S. troops uncovered. British officials have similarly accused Iran of meddling in Iraqi affairs, but say they have not found Iranian-made weapons in areas they patrol.
The lack of publicly disclosed evidence has led to questions about whether the administration is overstating its case. Some suggest Bush and his aides are pointing to Iran to deflect blame for U.S. setbacks in Iraq. Others suggest they are laying the foundation for a military strike against Iran.
Before invading Iraq, the administration warned repeatedly that Saddam Hussein was developing nuclear, chemical and biological weapons. Those statements proved wrong. The administration's charges about Iran sound uncomfortably familiar to some. "To be quite honest, I'm a little concerned that it's Iraq again," Sen. John D. Rockefeller IV, head of the Senate Intelligence Committee, said last week, referring to the administration's comments on Iran.
The accusations of Iranian meddling "illustrate what may be one of our greatest problems," said Anthony Cordesman, a former Defense Department official and military expert at the Center for Strategic and International Studies in Washington.
"We are still making arguments from authority without detail and explanation. We're making them in an America and in a world where we really don't have anything like the credibility we've had in the past."
Few doubt that Iran is seeking to extend its influence in Iraq. But the groups in Iraq that have received the most Iranian support are not those that have led attacks against U.S. forces. Instead, they are nominal U.S. allies.
The Supreme Council for Islamic Revolution in Iraq, one of the two largest parties in parliament, is believed to be the biggest beneficiary of Iranian help. The Shiite group was based in Iran during Hussein's reign, and Iran's Revolutionary Guard trained and equipped its Badr Brigade militia.
But the Supreme Council also has strong U.S. connections. Bush played host to the head of the party, Abdelaziz Hakim, at the White House in December, and administration officials have frequently cited Adel Abdul Mehdi, another party leader, as a person they would like to see as Iraq's prime minister.
The Islamic Dawa Party of Iraq's current prime minister, Nouri Maliki, also has strong ties to Iran.
Some U.S. officials have also suggested that Iran, a Shiite theocracy, has provided aid to the Sunni insurgents, who have led most of the attacks against U.S. forces. Private analysts and other U.S. officials doubt that. Evidence is stronger that the Iranians are supporting a Shiite group that has attacked U.S. forces, the Al Mahdi militia, which is loyal to radical cleric Muqtada Sadr.
Top U.S. intelligence officials have been making increasingly confident assertions about Iran.
"I've come to a much darker interpretation of Iranian actions in the past 12 to 18 months," CIA Director Michael V. Hayden said in recent congressional testimony. Previously, Tehran's priority was to maneuver for a stable Iraq dominated by its Shiite majority, but that attitude has changed, he said.
"There is a clear line of evidence that points out the Iranians want to punish the United States, hurt the United States in Iraq, tie down the United States in Iraq," he said.
One high-ranking intelligence official in Washington acknowledged a lack of "fidelity" in the intelligence on Iran's activities, saying reports are sometimes unclear because it is difficult to track weapons and personnel that might be flowing across the long and porous border.
But U.S. forces have picked up specially shaped charges used to make roadside bombs capable of penetrating advanced armor, he said, with markings that could be traced to Iran and dates that were recent. The markings have been found on the devices themselves or the crates in which they were smuggled into the country, he said.
"Two years ago we were debating whether this was really happening," the official said. "Now the debate is over."
U.S. officials have declined to provide documentation of seized Iranian ordnance despite repeated requests. The U.S. military often releases photographs of other weapons finds.
British government officials, including Prime Minister Tony Blair, have also accused Iran of supplying advanced explosive devices to Iraq.
Blair said a year ago that the weapons bore the hallmarks of Iran or Hezbollah, the Iranian-backed militia in Lebanon. But British officers stationed in Iraq at the time said they had seized no such weapons in the districts for which they had responsibility.
"We do have intelligence which suggests that weapons and ammunition are being smuggled in from Iran," Maj. David Gell, a spokesman for British forces in Basra, said last week. "We don't always manage to find any."
U.S. military officials in Diyala have had the same experience. No munitions or personnel have been seized at the border, officers said.
Sutherland, the U.S. colonel who oversees Diyala, believes that Tehran is prepared to work with any group, Shiite or Sunni, that can tie up U.S. forces. But State Department and intelligence officials have privately expressed doubts that Iranians are helping Sunnis.
Sunni insurgents in Diyala don't appear to need outside suppliers. They exploit massive weapons stashes containing materiel dating back to the Iran-Iraq war, when Hussein had a major military base in the area. U.S. military officials say they have found the type of shaped charges they attribute to Iran and Hezbollah in majority-Shiite parts of the province.
Outside military analysts have questioned how many of these sorts of weapons actually come from Iran. The technology used to make them is simple and widely known in the Middle East, they note. Iran is a likely source for some of the more sophisticated devices, but other countries could also be pitching in.
"A lot of rather sophisticated weapons have actually been released by Syria," said Peter Felstead, editor of the London-based Jane's Defense Weekly.
Others note that smugglers could be bringing weapons across the border from Iran without government approval.
'They are significant'
A second high-ranking U.S. intelligence official in Washington acknowledged that only a "small percentage" of explosions in Iraq could be linked to shaped charges coming from Iran.
"But in terms of American casualties, they are significant," he said, because they are much more lethal than standard roadside bombs.
A senior U.S. military intelligence official said coalition forces in Iraq had also found shaped charges "in the presence of Iranians captured in the country." He declined to elaborate but noted that U.S. operatives who raided an Iranian office in the Iraqi city of Irbil this month captured documents and computer drives he called a "treasure trove" on Iran's "networks, supply lines, sourcing and funding."
Five Iranians were taken into custody in the raid, prompting angry protests from the Iraqi government.
U.S. intelligence officials emphasized that Iran intentionally stops short of steps that would be seen as direct provocation and provide justification for a military response. For example, Iran has refrained from supplying Shiite militias with surface-to-air missiles and other weaponry that was part of Hezbollah's arsenal in its fight with Israel last summer, they said.
A high-ranking U.S. intelligence official called it a "careful calibration" that probably reflected disagreements within the Islamic regime. "I don't doubt that Iranian national security council meetings are very contentious," the official said.
Zavis reported from Baqubah and Miller from Washington. Times staff writers Peter Spiegel in Washington and Solomon Moore in Baghdad contributed to this report.