Labrador puppies “Hoey” (L) and “Hatton”, named in honor of September 11, 2001 attack victims Patrick Hoey and Lenny Hatton who died in the World Trade Center, are pictured on the grounds of the Pentagon near Washington, June 28, 2011. (Photo by Jason Reed/Reuters)





Pasha, a New Jersey Transit Police K-9 bomb dog, stays alert while resting at the Secaucus Junction Station, Friday, January 31, 2014, in Secaucus, N.J. (Photo by Julie Jacobson/AP Photo)





Boston Police Special Operations officers use a bomb-sniffing dog while searching a vehicle on a street near the federal courthouse, in Boston, Tuesday, March 3, 2015. A panel of 12 jurors and six alternates was seated Tuesday after two months of jury selection for the federal death penalty trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev. (Photo by Steven Senne/AP Photo)





In this Friday, July 1, 2011 photo a New York City police officer with a bomb sniffing dog patrols at a Times Square subway station in New York. Since terrorists brought down the twin towers on September 11, 2001, subways have been bombed across the world, including Madrid, London and this spring in Minsk. The possibility that New York’s sprawling, porous and famously gritty subway system could be next has become a constant worry. (Photo by Mary Altaffer/AP Photo)





Los Angeles Airport Police K-9 unit officer Heriberto Gonzalez and his dog Bady inspect passengers' items in a boarding area at Los Angeles International Airport December 29, 2009. (Photo by Mario Anzuoni/Reuters)





A member of the Boston Police K9 unit patrols with a bomb-sniffing dog at the finish line of the Boston Marathon in Boston, Saturday, April 18, 2015. The 119th Boston Marathon will be run on Monday. (Photo by Michael Dwyer/AP Photo)





A New York Police Department officer and a bomb sniffing dog patrol the finish line area before the start of the New York City Marathon in New York, November 3, 2013. In addition to cheering supporters, runners in this Sunday's New York City Marathon will also be monitored by bomb-sniffing dogs, police scuba divers and surveillance helicopters to prevent an attack similar to the one at the Boston race earlier this year. (Photo by Mike Segar/Reuters)





U.S. Park Police officer Dave Moen out out the New York field office praises his dog Blek as the pair practice detecting explosives at Lincoln Financial Field in Philadelphia, Wednesday, February 6, 2008. (Photo by Matt Rourke/AP Photo)





Miller, a Transit Police dog with the Massachusetts Transportation Authority Explosives Detection Unit, sniffs a bag at Back Bay Station as commuters enter the subway system a day after two explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 16, 2013. (Photo by Jessica Rinaldi/Reuters)





A police officer with a bomb-sniffing dog patrols the Times Square subway station, Friday, Sept. 9, 2011, in New York. Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials are chasing a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington. (Photo by Mary Altaffer/AP Photo)





New York City K-9 officer Anderson guides his bomb-sniffing dog through a corridor as people gather in Times Square in preparation for the New Year's Eve ball drop, Tuesday, December 31, 2013, in New York. (Photo by Kathy Willens)/AP Photo





A New York Police Department bomb-sniffing dog stands outside a portable latrine waiting for its officer during preparations for the 87th annual Macy's Thanksgiving Day Parade in New York, November 27, 2013. (Photo by Shannon Stapleton/Reuters)





An ATF agent and his bomb-sniffing dog patrols around the Warren E. Burger Federal Building where a detention hearing was being held Thursday, April 23, 2015, in federal court in St. Paul, Minn. for four of the six Minnesotans who are accused of plotting to travel to Syria to join the Islamic State group. Prosecutors have said they plan to ask a judge to keep the men in custody pending trial, saying they are a flight risk. (Photo by Jim Mone/AP Photo)





A police officer walks a dog on the steps of the South Carolina Statehouse as the state prepares to remove a Confederate battle flag from in front of the South Carolina Statehouse, Friday, July 10, 2015, in Columbia, S.C. South Carolina Gov. Nikki Haley signed a bill into law Thursday requiring the flag to be removed. (Photo by John Bazemore/AP Photo)