Sadr would be unlikely to be behind an attack on one of the Shi'ite's holy places. This is Zarqawi attempting to start civil war.
I wouldn't be surprised to see several more spectacular attacks in an attempt to ensure it.
Ah, but Sadar would have no problem doing that. I noticed on one thread they mentioned his men tried to get there by bus. Remember, Sadar took over the Iman Ali shrine in Najef and left behind bodies of innocent Shiia. Don't kid yourself about what Sadar would do to stir up trouble and to get America out of there.
I agree. Achams Razor applies here.