NEW : Magnetic bombs were used in other Iran assassinations , ambassador says . Secretary of State Hillary Clinton categorically denies U.S. involvement in the killing . Comment: A bomb was put under Mostafa Ahmadi Roshan 's car , state-run IRNA news agency says .
NEW : Magnetic bombs were used in other Iran assassinations , ambassador says . Secretary of State Hillary Clinton categorically denies U.S. involvement in the killing . An Israeli military spokesman says he does not know who carried out the attack . A bomb was put under Mostafa Ahmadi Roshan 's car , state-run IRNA news agency says .