Followers of Muqtada Sadr, a firebrand who scorns the US and israel and advocates an Islamic State are blamed for killing the London based Ayatollah Abdel Majid al-Koei and have also clashed with Sistani's people and with another faction, the Supreme Council for the Islamic Revolution in Iraq, whose leader Muhammad Baqr al hakimwas killed last August.
Followers of Muqtada Sadr would seem likely to be suspects in whatever happened and I would guess are backed by the Iranians!
Foxnews also had a clip on the assault and whether it did or did not happen.
I am watching for more info!