Whoever is responsible, the murders appear to be as much about spreading terror as they are about stopping the nuclear program. Iran has legions of capable engineers, and none of the victims appear to have had indispensable knowledge or abilities. According to the Congressional Research Service, Iran's main nuclear research complex in Isfahan probably has 3,000 employees alone, and the there are about 10 other major nuclear sites in the country.
But spreading fear among the living can slow them down, spread confusion, or deter young recruits. If scientists became frightened enough, they might be reluctant to travel to work or conferences inside the country. Meanwhile, enhanced security measures at the sites could prove cumbersome.
In November 2010, Majid Shahriari, a nuclear engineer at Shahid Beheshti University in Tehran, was killed by a car bomb. In July 2011, engineer Darioush Rezaeinejad, believed to be working within Iran's secretive nuclear program, was gunned down in the street. Last November, Maj. Gen. Hassan Moghaddam, considered the father of Iran's missile program, died with 16 others in a still unexplained explosion at a military base.