Iran-trained Shiite extremists back again

AP reports:

Groups of Shiite extremists trained in Iran are returning to Iraq with plans to bomb high-profile targets, the chief of Dhi Qar province's police said Saturday.

Brigadier General Sabah al-Fatlawi told AFP "Special Groups" (Shiite extremists) of around 10 fighters each are returning by crossing the border from Amara, the capital of Shiite Maysan province in the south.

How will Washington respond?