The slaying is sure to add to tension with the West, as Iran moves ahead with an atomic program that four rounds of U.N. sanctions have failed to slow.

The target, identified in Iranian media reports as 35-year-old Darioush Rezaei, was a physics professor whose area of expertise was neutron transport, which lies at the heart of nuclear chain reactions in reactors and bombs. One of the earlier assassinations, in November, killed a man with the same specialization.