Why are U.S. Forces Besieging Sadr City?

According to the Washington Post,

1. American forces believe some part of the Mahdi Army is responsible for capturing an Iraqi-American soldier. The soldier's brother, who was captured at the same time and released, said the Mahdi Army was responsible for their capture. U.S. forces are basically trying to get the American soldier back.
2. The U.S. military is trying to capture notorious death squad leader Abu Deraa, who has been claimed off and on by the Mahdi Army.

The U.S. looks like it has taken a stand it is hard to back down from without success. Either they get Abu Deraa or retreat like they did from the capture of Mohammad Farah Aidid in Mogadishu. And they either get the American soldier or admit they cannot save him.

The real question is whether the two operations are the same thing, overlap somewhat or are different parties of a larger strategy. Is the U.S. trying to capture Abu Deraa in order to get an American soldier back?