Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying, and the Obama administration condemned the attack, while provocative hints from Jerusalem reinforced the perception of an organized and clandestine campaign to set back Iran's nuclear ambitions.
The day before the attack, Israel's military chief, Lt. Gen. Benny Gantz, was quoted as telling a parliamentary panel that 2012 would be a "critical year" for Iran - in part because of "things that happen to it unnaturally."
The blast killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium-enrichment facility, the centerpiece of Iran's expanding program to make nuclear fuel. Roshan, 32, had planned to attend a memorial later Wednesday for another nuclear researcher who was killed in a similar pinpoint blast two years ago, Iranian media said.
"A heinous act," Iran's Atomic Energy Organization said of Wednesday's bombing.
The state news agency IRNA said Roshan had "organizational links" to Iran's nuclear agency, which suggests a direct role in key aspects of the program. Another news agency, the semiofficial Mehr, said Roshan had been interviewed by inspectors from the U.N. nuclear watchdog agency - which Iran has accused of placing its scientists in peril by including their names in public reports.
Natanz, in central Iran, is the country's main enrichment site. Officials said this week they were expanding some operations to an underground site with more advanced equipment.
The United States and its allies are pressuring Iran to halt uranium enrichment, a key element of the nuclear program that the West suspects is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel, but at higher levels it can be used as material for a nuclear warhead.
Iran denies it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity and producing medical radioisotopes to treat cancer patients.
The years of virtual stalemate between Iran and the West appear to be shifting into a new period of heightened pressures and tensions.
Russia strongly warned the West on Wednesday against any attack on Iran, saying it would upset global security.
Tehran has accused Israel's Mossad, the CIA, and Britain's spy agency of engaging in an underground "terrorism" campaign against nuclear-related targets, including at least three other slayings since early 2010 and the release of a malicious computer virus known as Stuxnet.