United Nations weapons inspectors have amassed a trove of new evidence that they say makes a “credible” case that “Iran has carried out activities relevant to the development of a nuclear device,” and that the project may still be under way.

The long-awaited report, released by the International Atomic Energy Agency on Tuesday, represents the strongest judgment the agency has issued in its decade-long struggle to pierce the secrecy surrounding the Iranian program. The findings, drawn from evidence of far greater scope and depth than the agency has previously made public, have already rekindled a debate among the Western allies and Israel about whether increased diplomatic pressure, sanctions, sabotage or military action could stop Iran’s program.

Knowing that their findings would be compared with the flawed Iraq intelligence that preceded the 2003 invasion — and has complicated American moves on Iran — the inspectors devoted a section of the report to “credibility of information.” The information was from more than 10 countries and from independent sources, they said; some was backed up by interviews with foreigners who had helped Iran.

The report laid out the case that Iran had moved far beyond the blackboard to create computer models of nuclear explosions in 2008 and 2009 and conducted experiments on nuclear triggers. It said the simulations focused on how shock waves from conventional explosives could compress the spherical fuel at the core of a nuclear device, which starts the chain reaction that ends in nuclear explosion.