This article is by Mark Mazzetti, Steven Lee Myers and Thom Shanker.

WASHINGTON  The United States has gathered its most detailed evidence so far of Iranian involvement in training and arming fighters in Iraq, officials say, but significant uncertainties remain about the extent of that involvement and the threat it poses to American and Iraqi forces.

Some intelligence and administration officials said Iran seemed to have carefully calibrated its involvement in Iraq over the last year, in contrast to what President Bush and other American officials have publicly portrayed as an intensified Iranian role.

It remains difficult to draw firm conclusions about the ebb and flow of Iranian arms into Iraq, and the Bush administration has not produced its most recent evidence.

But interviews with more than two dozen military, intelligence and administration officials showed that while shipments of arms had continued in recent months despite an official Iranian pledge to stop the weapons flow, they had not necessarily increased.