Updated at 2:43 p.m. ET
TEHRAN, Iran - An Iranian news website is reporting several suspects have been arrested over last week's killing of an Iranian nuclear scientist.
Ali Larijani, speaker of the Iranian parliament, said the suspects are being interrogated, and the investigation is continuing. He talked to Iran's state Arabic language TV channel Al-Alam, and his comments were carried on the Tabnak.ir site.
Mostafa Ahmadi Roshan, an official in Iran's nuclear enrichment facility in Natanz, was killed in a brazen daylight assassination Wednesday when two assailants on a motorcycle attached a magnetic bomb to his car in the Iranian capital. The killing bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program, and has prompted calls in Iran for retaliation against those deemed responsible.
Iran accused the U.S., Britain and Israel of involvement.
Washington denied any role in the assassination, and London condemned the killing of civilians.
Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement.
The U.S. and its allies fear Iran's program aims to develop nuclear weapons. Iran denies the charges, and says its nuclear program is for peaceful purposes only.
On Saturday, Iran said it has evidence that the United States was behind the assassination.
The IRNA state news agency said that Iran's Foreign Ministry had sent a diplomatic letter to the U.S. saying that it has "evidence and reliable information" that the CIA provided "guidance, support and planning" to assassins "directly involved" in Roshan's killing.
Iran delivered the letter to the Swiss Embassy in Tehran, which looks after U.S. interests in the country. Iran and the U.S. have had no diplomatic relations since the 1979 Islamic Revolution.
IRNA also reported that Iran delivered a letter to Britain accusing London of having an "obvious role" in the killing. It said that a series of assassinations began after British intelligence chief John Sawers hinted in 2010 at intelligence operations against the Islamic Republic.
British media have quoted Sawers as saying that intelligence-led operations were needed to make it more difficult for countries like Iran to develop nuclear weapons.
The killing has sparked outrage in Iran, and state TV broadcast footage Saturday of hundreds of students marching in Tehran to condemn Roshan's death and calling for the continuation of the country's disputed nuclear program.
In the clearest sign yet that Iran is preparing to strike back for Roshan's killing, Gen. Masoud Jazayeri, the spokesman for Iran's Joint Armed Forces Staff, was quoted by the semiofficial ISNA news agency Saturday as saying that Tehran was "reviewing the punishment" of "behind-the-scene elements" involved in the assassination.
"Iran's response will be a tormenting one for supporters of state terrorism," he said, without elaborating. "The enemies of the Iranian nation, especially the United States, Britain and the Zionist regime, or Israel, have to be held responsible for their activities."
Jazayeri also accused the International Atomic Energy Agency of being partially to blame, saying that the U.N. nuclear watchdog made public a list of Iranian nuclear scientists and officials that "has provided the possibility of their identification and targeting by spy networks."