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Nick Dowling, a former National Security Council official and president of the security consulting firm IDS International, said “there is too much bluster and tension in the U.S.-Iran relationship at this point to take anything at face value.”

Iran is fast to point fingers at Israel and the United States, but they have not produced any evidence,” Mr. Dowling said. “U.S. and Israeli leaders deny involvement. Iran certainly wants to play the victim here and portray Israel and the U.S. as provocateurs.”

The bluster was on display last week when Iran threatened to blockade the Strait of Hormuz, through which one-sixth of the world’s oil is transported.

“The provocative rhetoric coming out of Iran in the last week has been quite concerning,” Mrs. Clinton said Wednesday, adding that the Strait of Hormuz is an “international waterway.”

“The United States and others are committed to keeping it open,” she said.

Mrs. Clinton also said that “there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for it to end its provocative behavior, end its search for nuclear weapons and rejoin the international community and be a productive member of it.”

Yesterday’s bombing was the fourth of its kind over the past two years and very nearly coincided the two-year anniversary of a car bombing that killed Masoud Ali Mohammadi, a physics professor at Tehran University, on Jan. 12, 2010.

Majid Shahriari, a nuclear engineer for the Atomic Energy Commission, was killed in a car-bomb attack on Nov. 29, 2010. Fereydoun Abbasi, a nuclear scientist at Shahid Beheshti University, was wounded in a similar attack the same day.

Darioush Rezai-Nejad, a nuclear scientist at K.N. Toosi University of Technology in Tehran, was fatally shot in Tehran on July 23, 2011. His wife also was killed in that attack.

Abraham Rabinovich contributed to this report.