An Iranian nuclear scientist has been killed and another wounded in two separate but similar attacks in the capital, Tehran.
The scientists were targeted by men on motorbikes who attached bombs to the windows of their cars as they drove to work, officials said.
The scientist killed has been named as Majid Shahriari.
President Mahmoud Ahmadinejad has accused "Western governments" and Israel of being behind the killing.
Another scientist was killed in a bomb blast at the beginning of the year.
Dr Shahriari was a member of the nuclear engineering department of Shahid Beheshti University in Tehran. His wife is said to have been injured in the attack.
The nuclear scientist injured in the second attack was named as Fereydoon Abbasi. His wife was also wounded.
According to the conservative news website Mashregh News, Dr Abbasi is "one of the few specialists who can separate isotopes" - a process that is crucial in the manufacture of uranium fuel for nuclear power stations and is also required for the creation of uranium-based nuclear weapons.
Dr Abbasi has also been a member of the Revolutionary Guards since the 1979 revolution, the website said.
At a news conference, President Ahmadinejad accused Western powers and Israel of being behind the murder.
"One can undoubtedly see the hands of Israel and Western governments in the assassination which unfortunately took place," he said, without specifying which Western governments.
He said the assassination would not stop Iran from pursuing its nuclear programmes.
Earlier, state television reported a similar claim by Iran's Interior Minister, Mostafa Mohammad Najjar, who accused US and Israeli intelligence services of killing the scientist.
"Mossad and the CIA are the enemies of Iranians and always seek to hurt this nation. They particularly want to stop our scientific progress," he said.
The head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, who went to visit the surviving scientist in hospital, said he had a message for the country's enemies: "Do not play with fire".
The Iranian scientist killed in January this year, Masoud Ali Mohammadi, was said to be a nuclear scientist assassinated by counter-revolutionaries, Zionists and agents of the "global arrogance", Iranian media said at the time.
But scientists in the UK and the US said that, from his substantial body of published research, Dr Mohammadi was unlikely to have been working on Iran's nuclear programme, and that his expertise was in another field of physics altogether - quantum mechanics.
There has been much controversy over Iran's nuclear activities.
Tehran says its nuclear programme is for peaceful energy purposes, but the US and other Western nations suspect it of seeking to build nuclear weapons.
On Saturday, Iran said its first atomic power plant in the southern city of Bushehr had begun operations, ahead of a new round of talks with Western powers over the country's nuclear drive.