"The main elements behind the killings...were arrested and moved to detention following an investigation of at least 18 months involving surveillance in Iran and abroad,” said a statement from the ministry published by state media.

The detainees are believed to be involved in the murders of Majid Shahriari, a key member of Iran's Atomic Energy Organization and Mostafa Ahmadi Roshan, a deputy director of Iran’s largest uranium enrichment plant Natanz .

Early on November 29, 2010, Shahriari was killed and another, nuclear scientist Fereydoun Abbasi, was seriously wounded in Tehran. These were copycat operations, timed for rush hour (7:30 a.m.) in different parts of the city: motorcycle riders stuck directional bombs with shrapnel to the windows of the scientists' cars and managed a quick getaway five to seven seconds before the ensuing blasts.

Roshan and his bodyguard died in a car bomb explosion in Tehran on January 11, 2012. Local police sources said a passing motorcyclist attached a magnetic mine to the car the scientist was riding in.

Iran earlier blamed these and two other murders of Iranian scientists on Israeli and U.S. intelligence.

Western nations suspect Iran of seeking to build nuclear weapons under the guise of its nuclear program, which Tehran says is aimed at the peaceful generation of civilian energy.