TEHRAN, Iran (AP) — Two assailants on a motorcycle attached a magnetic bomb to the car of an Iranian university professor working at a key nuclear facility, killing him and his driver Wednesday, reports said. The slayings suggest a widening covert effort to set back Iran's atomic program. The attack in Tehran bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program. It is certain to amplify authorities' claims of clandestine operations by Western powers and their allies to halt Iran's nuclear advances. The blast killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, state TV reported. 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. Natanz is Iran's main enrichment site, but officials claimed earlier this week that they are expanding some operations to an underground site south of Tehran with more advanced equipment. The U.S. 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. 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 slayings since early 2010 and the release of a malicious computer virus known at Stuxnet in 2010 that temporarily disrupted controls of some centrifuges — a key component in nuclear fuel production. All three countries have denied the Iranian accusations. Israeli officials, however, have hinted about covert campaigns against Iran without directly admitting involvement. I bet they were watching for Israel's planes to drop bombs too much and not watching the ground .