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)

In this September 14, 2017 photo, young women pose for a photo inside their squatter building that used to house the Brazilian Institute of Geography and Statistics (IBGE) in the Mangueira slum of Rio de Janeiro, Brazil. Many lower-middle class Brazilians who gained ground during the boom years have since slid back closer to the poverty line. (Photo by Felipe Dana/AP Photo)

Houthi followers perform the traditional Baraa dance ahead of a demonstration against the Saudi-led air strikes in Yemen's capital Sanaa August 24, 2015. The nothern-based Houthis, a Shi'ite Muslim group, took control of Sanaa last September. Arab countries intervened in the conflict in March to halt a Houthi advance into the south which caused the Saudi-backed government to flee to Riyadh from its refuge in the southern port of Aden. (Photo by Khaled Abdullah/Reuters)

The Mount Bromo volcano erupts in the Bryce Canyon, Utah. (Photo by Reynold Dewantara/2016 National Geographic Travel Photographer of the Year Contest)

Cyclists desperately try to keep their heavily-laden bicycles upright as they arrive at a market with baskets full of pineapples on August 23, 2018. The men travel up to 12 and a half miles with two baskets tied to the sides of their bikes, carrying between 50 and 100 pineapples to sell. Each of the bicycles is so heavily laden with fruit it is impossible for the men to actually ride their bikes, instead having to walk alongside them. When they arrive at the market place in Madhupur, Bangladesh, buyers will pay up to 30 Taka for a pineapple – the equivalent of around 28 pence. (Photo by Abdul Momin/Solent News & Photo Agency UK)

Masked guests attend the “Grand Bal Christian Dior” during the Haute Couture Spring-Summer 2018 fashion collection presented in Paris, Monday, January 22, 2018. (Photo by Kamil Zihnioglu/AP Photo)

Revellers take part in New Year celebrations in Coin, near Malaga, southern Spain, January 1, 2016. (Photo by Jon Nazca/Reuters)

Nazi dictator Adolf Hitler greets supporters at a May Day gathering in the Olympic Stadium, Berlin. (Photo by Keystone). 1st May 1938