Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Concerns about westerners joining the fight in Syria dominated the annual Aspen National Security Forum with top U.S. government officials equating the threat from the stalemated Syrian civil war to that posed by al Qaeda's Yemen affiliate. Michael Vickers, the Pentagon's undersecretary for intelligence, said there are now believed to be thousands of western passport holders, many with easy visa access to enter the United States, who are among foreign fighters in Syria. California man arrested, said he wanted to join ISIS John Carlin, the Justice Department's national security chief, called the threat "unprecedented" and said the department is putting resources to prosecute Americans who try to travel to the region to join terror groups involved in the war.Annual Aspen Security Conference brings together top national security figuresDiscussion tends to trend toward current eventsTSA administrator says improvised explosives more of a concern than shoot-downsThere might be a less-publicized reason that 2009 underwear bomb didn't detonate 
 Question:He cited the example of a new iteration of the underwear bomb that prompted _ to change screening equipment settings and retrain explosives detection dogs.

TSA