WASHINGTON — An imminent report by United Nations weapons inspectors includes the strongest evidence yet that Iran has worked in recent years on a kind of sophisticated explosives technology that is primarily used to trigger a nuclear weapon, according to Western officials who have been briefed on the intelligence.

But the case is hardly conclusive. Iran’s restrictions on inspectors have muddied the picture. And however suggestive the evidence about what the International Atomic Energy Agency calls “possible military dimensions” of Iran’s program turns out to be, the only sure bet is that the mix of sleuthing, logic and intuition by nuclear investigators will be endlessly compared with the American intelligence agencies’ huge mistakes in Iraq in 2003.

Just as it was eight years ago, the I.A.E.A., which was conceived as a purely technical organization insulated from politics, is about to be sucked into the political whirlpool about how the world should respond to murky weapons intelligence. Except this time everything is backward: It is the I.A.E.A., which punched holes in the Bush administration’s claims about Iraq’s nuclear progress, that today is escalating the case that Iran has resumed work on bomb-related technology, after years of frustration over questions that have gone unanswered by that government.

For its part, the Obama administration, acutely aware of how what happened in Iraq undercut American credibility, is deliberately taking a back seat, eager to make the conclusions entirely the I.A.E.A.’s, even as it continues to press for more international sanctions against Iran. When the director of the agency, Yukiya Amano, came to the White House 11 days ago to meet top officials of the National Security Council about the coming report, the administration declined to even confirm he had ever walked into the building.