Instead of Fereidoun Abbasi, Iran's nuclear chief and a deputy president, or one of his senior aides, the Iranian delegation at the talks was headed by Ali Asghar Soltanieh, Tehran's chief IAEA delegate, they said.
The Vienna-based agency has regular access to Soltanieh and the Iranian decision to have him lead his country's negotiations could mean the Islamic Republic did not view the meeting as a top priority.
Diplomats differed on how effective this week's talks were. One said it was essentially a failure because the IAEA experts failed to dent more than three years of Iranian stonewalling on what the agency says is a growing list of evidence that Iran has worked or is working on nuclear arms.
But thee others took a longer view.
They said the IAEA team refused to accept an Iranian plan that would have limited the scope of its probe. Instead, the agency team left Tehran only after giving the Iranian side a concrete IAEA blueprint that would commit Tehran to answer all questions on the suspicions.
The next mission to Tehran will begin Feb. 19. If Iran rejects the proposals and continues to insist that it is innocent, the stage is set for further international action first at a meeting of the IAEA's 35-nation board that meets in March, and then potentially by the U.N. Security Council.
The board's 2006 referral of Iran to the U.N. Security Council for its refusal to rein in its nuclear program led to the first of the now four sets of U.N. sanctions on the Islamic Republic, along with a growing list of other penalties by Washington and its allies.
Iran remains publicly defiant. Supreme leader Ayatollah Ali Khamenei pledged Friday to aid any nation or group that challenges Israel and said military strikes over the Islamic Republic's nuclear program would damage U.S. interests in the Middle East "10 times over."
It also launched a satellite that it said was meant to gather weather data but which raises concerns because of its possible military applications.
A British Foreign Office statement linked the launch to Iran's ambitious missile program, noting that U.N. Security Council resolutions prohibit Iran from "launches using ballistic missile technology," and adding: "The technology used in launching this satellite incorporates that used in ballistic missiles."
Still, Tehran may feel it is under the gun to defuse nuclear tensions with the most recent U.S. and European sanctions targeting Iran's oil exports Tehran's lifeline and fears high of an Israeli attack on its nuclear installation.
The Iranians are "acutely aware that they cannot come back and say the same thing in three weeks and that puts a lot of pressure on them," said one of the diplomats, who was briefed on the IAEA trip.
Iran insists the allegations of secret weapons work are based on documents faked by the U.S., Israel and their allies.
Faced with Iranian refusal to engage on the issue, the IAEA summarized what it knows in November in a 13-page document drawing on 1,000 pages of intelligence. It stated then for the first time that some of the alleged experiments can have no other purpose than developing nuclear weapons.
The IAEA team wants to talk to key Iranian scientists suspected of working on a weapons program. They also seek to inspect documents related to nuclear work and secure commitments from Iranian authorities to allow future visits.
Washington and its allies also want Iran to halt uranium enrichment, which they believe could eventually lead to weapons-grade material and the production of nuclear weapons. Iran says its program is for peaceful purposes generating electricity and producing medical radioisotopes to treat cancer patients.
Since the discovery in 2002 that Iran was secretly working on uranium enrichment, the nation has expanded that operation to the point where it has thousands of centrifuges churning out enriched material the potential source of both nuclear fuel and fissile warhead material.
Iran also has started producing uranium at a higher level than its main stockpile a move that would jump start the creation of highly enriched, weapons grade uranium, should it chose to go that route. And it is moving its higher-enriched operation into an underground bunker that it says is safe from attack.
David Stringer contributed to this report from London.
George Jahn can be reached at http://twitter.com/georgejahn