FILE - In this Sept. 11, 2011 file photo, Secret Service snipers keep watch from the roof of the Sept. 11 Memorial Museum during the ceremony to mark the 10th anniversary of the attacks on the World Trade Center in New York. A Las Vegas shooting from a high-rise hotel that killed dozens of people in a packed concert below on Sunday, Oct. 1, 2017, has forced other cities to examine their tactics for dealing with this kind of nightmare scenario. In New York, which hosts Times Square New Year's Eve and other events surrounded by high-rises, police say they use rooftop snipers to scan for threats, and make security sweeps of nearby hotels. (AP Photo/Timothy A. Clary, Pool, File)
Tributes to 9/11 victims across the U.S.
Tributes were held around the country for the 10th anniversary of the Sept. 11 terrorist attacks on the U.S.