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Nevertheless, intelligence officials contacted by NEWSWEEK insist that Iranian interference in Iraq appears significant. U.S. intelligence officials say they are aware of staging points—sometimes upgraded in unofficial accounts to "training camps"—in Iran, Syria and Lebanon that are used by insurgents traveling in and out of Iraq. Intelligence agencies believe that supplies, such as the sensors, are shipped from these locations.

One U.S. official said that the staging posts do not necessarily stay in the same location for long, which would greatly complicate any efforts by U.S. secret military units or intelligence teams to shut them down. Another U.S. official said that information about one such staging area in Syria is well known. But the details about what goes on there—and information about the extent to which civilians are present at the site—are murky. Any U.S. move against them could cause unwanted civilian casualties and a major confrontation with Syria.

U.S. officials believe supplies and personnel are moved across the Iraqi border from Iran via "ratlines"—intelligence jargon for smuggling routes—which, in some cases, are fairly well-documented by intelligence reporting. However, those same routes are also used by large numbers of Iranian pilgrims who travel to and from sacred Shia shrines inside Iraq. This can make it difficult, if not impossible, for U.S. forces to crack down on Iranian aid to insurgents because it is impossible to tell pilgrims from Iranian government operatives.

U.S. officials note that all the major Iraqi Shiite parties, including Prime Minister Nuri al-Maliki’s, have strong historical ties to Iran; some of the groups even operated from Tehran during Saddam Hussein's dictatorship. An Iranian outpost in the Kurdish town of Erbil, which was recently the target of a controversial U.S. military raid, has been known for the last 10 years as an Iranian intelligence base, a former official said. And one of the biggest open secrets in Iraq is the identity of the station chief of Iranian intelligence. According to former CIA official Riedel, he is one of the "most important figures in Baghdad." But this person operates from the Iranian Embassy, almost certainly under cover of diplomatic immunity. While his identity is known to intelligence agencies and his activities may be watched, there is little if anything—short of a violation of diplomatic conventions that could amount to an act of war—that U.S. forces can do to shut him down.

So, essentially, we are helpless to stop any of this. We don't want confrontation. We don't want civilian casualties. We don't want to violate diplomatic conventions with a nation that is famous for doing so.