Are Air Marshals effective?

What's the proper role of an air marshal? 

One testified Tuesday before a U.S. Senate Homeland Security Committee, and he had some interesting things to say, according to Tweets from Washington Post reporter Ashley Halsey. 

The Air Marshal was Robert J. MacLean, who was fired for speaking candidly about problems with the program in 2003. He won his U.S. Supreme Court case last month and was recently reinstated. 

Here are some of Halsey's tweets from the hearing, which also included three other senior security officials. A lot of the hearing focused on the recent TSA leak - someone made public a report saying screeners miss a lot of weapons - but I prefer the Air Marshal stuff.