He is suspected of running the operations of suicide bombers and kidnappings/torture/beheadings of USA Military and Iraqis from Syria. Prime Minister Ayad Allawi accused the Syrian government of harboring these terrorists back in December. He has declared that all of them will be brought to justice.
Many more were captured, and many more were murdered. To see the whole story, [read more].
UPDATE: Prime Minister Allawi is giving credit to the Syrian government for their part in securing the capture of al-Hassan. Either Syria is scared, finally getting the message that we mean business, or he's got something on Allawi. The last one I doubt. I do believe Allawi is a man that wants peace is his region, however, and may be trying to diplomatically play the goodwill card.