Deputy Defense Secretary William Lynn (left) presents Gen. David Petraeus with an American flag at his retirement ceremony after 37 years in the Army, in Arlington, Va., on Wednesday. Petraeus will now lead the CIA. Saul Loeb/AFP/Getty Images hide caption
A newspaper left by visitors on May 2 in Shanksville, Pa., at the fence surrounding the crash site of Flight 93. The chairmen of the 9/11 Commission say the U.S. has improved security over the past decade, but gaps in the system remain. Jeff Swensen/Getty Images hide caption
A K-9 police officer and his partner, "Bart," patrol New York's Grand Central Terminal in 2003. Less visible are the clandestine security measures the government has implemented since 2001. Joe Kohen/AP hide caption
NYPD Deputy Commissioner for Intelligence David Cohen (far left) stands with other city officials to announce a foiled terrorist plot against a New York synagogue on May 12. Mary Altaffer/AP hide caption
James Cromitie, center, is led by police officers from a federal building in New York, Thursday, May 21, 2009, after being arrested on charges related to a bombing plot in the Bronx. The arrest of Cromitie and three other Muslim ex-convicts in the alleged homegrown terror plot is renewing fears about the spread of Islamic extremism in the nation's prisons. (AP Photo/Robert Mecea) Robert Mecea/ASSOCIATED PRESS hide caption
This image provided by the Drug Enforcement Administration shows Russian arms trafficking suspect Viktor Bout in U.S. custody after being extradited and flown from Bangkok to New York to face terrorism charges. DEA/AP hide caption
Osama bin Laden (left) and his top lieutenant, Egyptian Ayman al-Zawahiri, are seen at an undisclosed location in this television image broadcast Oct. 7, 2001. In Counterstrike, reporters Thom Shanker and Eric Schmitt say the United States tried to disrupt al-Qaida by attacking the organization's financiers and middlemen. AP hide caption
A TSA employee being trained in behavioral pattern recognition watches passengers in line at Boston's Logan International Airport in 2010. This week, Logan will become the first U.S. airport to require every passenger to go through behavioral profiling. Josh Reynolds/AP hide caption
BRAC Commission Chairman Anthony J. Principi, and other member of the commission raise their hands in favor of closing Walter Reed Army Medical Center in Washington during a base closing hearing Aug. 25, 2005 in Arlington, Va. Mark Wilson/Getty Images hide caption
Adm. Eric Olson, former commander of the U.S. Special Operations Command, testifies before the House Armed Services Committee in 2009. Tim Sloan/AFP/Getty Images hide caption
The 30 American service members who died on Saturday were aboard a Chinook helicopter like this one when it was hit by a rocket-propelled grenade. Romeo Gacad/AFP/Getty Images hide caption