This was a professional hit. The scientist was killed while his 4 year old daughter was uninjured. Broad daylight. Busy street. At a kindergarten.

Financial Times:

On Saturday, gunmen riding motorcycles shot dead Darioush Rezaei-Nejad, a 35-year-old who was studying for masters degree in electronics, in front of a kindergarten in east Tehran where the victim and his wife were picking up their daughter.

Ms Rezaei-Nejad was taken to hospital after sustaining injuries while the four-year-old daughter was unscathed.

No arrests have been made in connection with the killing but Ali Larijani, the speaker of Iran's parliament, blamed the US for the assassination of "one of the country's elite". The higher education ministry issued a statement accusing "the agents of the global arrogance", a reference to the US and Israel.

Besides the US and Israel, another suspect would be...the Iranian government. The motorcyle hit is reminiscent of the Basij, although their level of expertise is limited. If he crossed somebody in government - or if his loyalty was suspect - the Revolutionary Guards might have been ordered to take him out. The Rev guards have assumed responsibility for guarding nuke scientists, so who better to take him out than his protectors? 

Still, the prime suspects would have to be the Israelis. The have the motive, the means, and certainly the opportunity.

This is the 4th attack on nuclear scientists in the last two years.