According to an article here U.S. officials confirm that a dissident Iranian group the People's Mujahedin of Iran (referred to by various acronyms, MEK, MKO and PMI), is supported by Israel. The U.S. has long designated the MEK as a terrorist group. At one time they were allied with Iran and supported the takeover of the U.S. embassy in Iran. However, they broke with the regime in 1980 and have been accused of terrorist attacks against Iran since.
Terrorist attacks have killed a total of 5 Iranian scientists since 2007 and the MEK may also have destroyed a missile development and research center. Iranian officials have claimed that there is a close connection between the MEK and Israel. However, U.S. officials now seem to confirm this.
The MEK renounced violence years ago. Two senior U.S. officials have confirmed in an NBC interview the MEK role in assassinations but both deny any U.S. involvement. Iranian authorities claim that Israel actually trains MEK agents. Analysts indicate that the skill with which the attacks take place shows the hand of a clever intelligence agency. Mossad would fit that description!
For much more detail with description of the attacks see the entire article. An Israeli spokesperson said:
"As long as we can't see all the evidence being claimed by NBC, the Foreign Ministry won't react to every gossip and report being published worldwide." The MEK denies it is involved in any of the attacks. Whoever is responsible the attacks are definitely happening and if Israel (or the U.S.) is providing support than that support would be for terrorism. The MEK has a number of supporters in the west who would like to see them removed from the official list of terrorist groups.