One problem for President Obama and his administration is that since American intelligence claims about Iraq’s illicit weapons proved false in 2003, assertions by the United States about its adversaries have routinely faced skepticism from other countries.

“Of course, that is in people’s heads. Everyone is extremely skeptical about U.S. intelligence revelations,” said Volker Perthes, an Iran expert who is the director of the German Institute for International and Security Affairs in Berlin.

There may indeed have been a plot, Mr. Perthes said. “I don’t regard it as impossible but rather improbable,” he said.

If the Iranian leadership authorized such a plot, he said, that would mark “a major escalation against the United States, of the kind that hasn’t happened since the Iranian Revolution.” It would be “almost an act of war,” and Washington “must react in a different way than it has so far.”

Alain Frachon, a Le Monde columnist and a former Washington correspondent, said that “we can expect anything from a regime as split and divided as the Iranian regime is,” adding that “one group among them is probably capable of launching such an operation to embarrass the others.”

While the United States’ history with Iraq might color the European reaction, Mr. Frachon said, he is “not sure you’ll find the same amount of skepticism in Paris as there was with W.M.D. In the case of Iraq, it was easier to assess, but Iran is much more opaque, and people are willing to expect anything from a divided regime.”