Iranian security forces stand guard around the site of an explosion that killed Mostafa Ahmadi Roshan, a chemistry expert and a director of Iran's Natanz uranium enrichment facility, in Tehran on Wednesday, Jan. 11, 2012. (AP Photo/International Iran Photo Agency, Sajjad Safari)

Photo by: Sajjad Safari

Iranian security forces stand guard around the site of an explosion that killed Mostafa Ahmadi Roshan, a chemistry expert and a director of Iran's Natanz uranium enrichment facility, in Tehran on Wednesday, Jan. 11, 2012. (AP Photo/International Iran Photo Agency, Sajjad Safari)