From New York City to San Francisco, bomb-sniffing dogs are at work, busily sniffing out at-risk public locations for explosives, following yesterday’s deadly bombings at the Boston Marathon.

In Boston itself, the police, tactical squads, members of the National Guard and numerous dogs are all combing the city and surrounding areas for suspicious devices, clues to yesterday’s tragedy and other security threats, according to the Boston Herald.

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Reports are also now coming in that bomb-sniffing dogs, as well as law enforcement spotters on roofs, were present at the start of the Boston Marathon. Such dogs were even said to be present at the finish line of the race.

The most elite bomb-sniffing dogs must learn to recognize thousands of active ingredients that might be used in an explosive. The dogs are exposed to various compounds and, using classical conditioning techniques, are trained to recognize them. As an article in Slate points out, “C-4 has an incredibly strong scent; it’s followed in decreasing order of smelliness by dynamite, TOVEX, detonating cord, and TNT.”

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Details about the explosive devices used in Boston are still coming in. Representative Ted Poe (R-Texas) said in an interview that the devices were similar to IEDs used in Afghanistan. They were full of ball bearings, which can tear into people like bullets.

Outside of the Northeastern region, bomb-sniffing dogs were deployed yesterday at certain landmarks, government buildings, transit hubs and sporting events. In California, for example, more such dogs are now on patrol inside and outside of Los Angeles International Airport and SFO in San Francisco.

Detroit, Las Vegas, Atlanta, Seattle and most major cities nationwide are also reviewing their own security measures and stepping up police and police dog presence.

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Outside of the United States, there is particular concern in London due to Wednesday’s planned ceremonial funeral for former Prime Minister Margaret Thatcher as well as Sunday’s London Marathon. You can count on numerous bomb-sniffing dogs being deployed at both events.

To learn more about how bomb-sniffing dogs work, please check out this How Stuff Works primer.

(Image: Tucson PD bomb-sniffing dog; Credit: Bill Morrow)