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Tom Gjelten

Pope Francis arrives in Washington, D.C., this afternoon for his first trip to the U.S. Here's a look at some of the key moments to watch during the leader of the Roman Catholic Church's six-day visit.

Tuesday: The President Greets The Pope

The Church of Scientology is famous for its efforts to silence its critics, but it has not blocked an upcoming HBO film that turns a harsh light on the powerful organization and its leadership.

Going Clear: Scientology and the Prison of Belief, directed by Academy Award-winning documentarian Alex Gibney, will debut Sunday over the vigorous objection of Scientology officials.

As investigators look into the Boston Marathon bombings, one crucial question is whether the suspects, brothers Tamerlan and Dzhokhar Tsarnaev, acted alone or had help. The clues might be found in the bombs used.

From what is now known, it appears the brothers assembled a whole arsenal of explosives. Watertown Police Chief Edward Deveau told CNN last weekend that the suspects had at least six bombs, including the two used in the attack and one thrown at police during a shootout.