NEW : Magnetic bombs were used in other Iran assassinations , ambassador says . They emphasized the need for a negotiated solution to the standoff . ` 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 . Comment: A bomb was put under Mostafa Ahmadi Roshan 's car , state-run IRNA news agency says .