TEHRAN, IRAN — It seemed a clockwork killing: Motorcycle riders flashed by and attached a magnetic bomb onto a car carrying a nuclear scientist working at Iran’s main uranium enrichment facility. By the time the blast tore apart the silver Peugeot, the bike was blocks away, weaving through Tehran traffic after what Iran calls the latest strike in an escalating covert war.

The attack, which instantly killed the scientist and fatally wounded his driver on Wednesday, was at least the fourth targeted hit against a member of Iran’s nuclear brain trust in two years. Tehran quickly blamed Israeli-linked agents backed by the U.S. and Britain.

Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying, and the Obama administration condemned the attack. However, 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, Israeli 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,” said Iran’s Atomic Energy Organization of Wednesday’s bombing.

It added a tone of defiance. “We will continue our (nuclear) path without any doubt. … Our path is irreversible,” said the statement carried on state television.

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 south of Tehran with more advanced equipment.