An unknown number of guns are being built with the mechanisms, causing problems for the police and ATF officials. Photo: An ATF agent holds a generic unfinished receiver, back, and one that has been manufactured, front, at an ATF field office in Washington, on May 06, 2014. The ATF is trying to crack down on the trade in the makeshift guns by targeting shops and individuals who offer to turn the unfinished receivers into functional pieces for firearms. (Photo by Bill O'Leary/The Washington Post)

A protester is sprayed with mace by riot police after throwing a chair through a window of a business during the second night of demonstrations after a not guilty verdict in the murder trial of former St. Louis police officer Jason Stockley, charged with the 2011 shooting of Anthony Lamar Smith, who was black, in St. Louis, Missouri, U.S., September 16, 2017. (Photo by Lawrence Bryant/Reuters)

An incredible view of Thunderstorms light up the insides of clouds near a beautiful sky line. This is truly heavenly weather as pictures taken from an airplane cockpit reveal what pilots see from above. It looks like at cruising altitude the weather really hots up, with the flight deck revealing some amazing scenes. Thunderstorms light up the insides of clouds, lightening streaks across the sky like cracks in a windscreen, the northern lights sweep uninterrupted across the sky and the galaxy stretches on forever. The pictures were captured by senior first officer Christiaan van Heijst, a 33-year-old from the Netherlands, and his friend Daan Krans. (Photo by Christiaan van Heijst/Daan Krans/Caters News Agency)

Activists and supporters of Ukrainian nationalist groups light flares during a rally to mark the third anniversary of the Euromaidan Revolution in downtown Kiev, Ukraine, 21 November 2016. About one thousand activists took park in the rally during which they burned tires on the Independence Square, destroyed the Russian bank Sberbank and broke windows in the office of a Ukrainian politician Viktor Medvedchuk, according to Ukrainian media. (Photo by Roman Pilipey/EPA)

Kale grows at Kajodlingen farm in Gothenburg, Sweden, September 28, 2016. They are doing it on the rooftops, on tower block balconies and even on a disused railway: Swedes have discovered a passion for urban gardening as a way of growing fresh food and getting back in touch with nature. Part of a global movement, an increasing number of Swedish city-dwellers are growing their own in window boxes and allotments or are visiting public gardens built in or on industrial or office spaces. (Photo by Maxim Shemetov/Reuters)

A young moose stands behind a window in an administration building of Siemens in Dresden, Germany Monday August 25, 2014. Police are trying to capture a moose on the loose in the eastern German city of Dresden. A spokesman for Dresden police says the young bull walked into the offices of German industrial giant Siemens on Monday and got stuck behind a glass wall. Marko Laske says officers and wildlife are trying to shoo the moose into a container so he can be taken to the local zoo. Moose are rare in Germany and the animal is likely to have come from neighboring Poland. (Photo by Arno Burgi/AP Photo/DPA)

Waves triggered by Typhoon Hato are seen in Hong Kong, China on August 23, 2017. A powerful typhoon barreled into Hong Kong on Wednesday, forcing offices and schools to close and leaving flooded streets, shattered windows and hundreds of canceled flights in its wake. Typhoon Hato came within 60 kilometers (37 miles) of Hong Kong, close enough to be considered a direct hit under Hong Kong's storm warning system. It was headed toward the western side of mainland China's Pearl River Delta. (Photo by Tyrone Siu/Reuters)