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)

Hope Solo of the USA looks dejected after losing 3-5 after penalty shoot-out the FIFA Women's World Cup Final match between Japan and USA at the FIFA World Cup stadium Frankfurt on July 17, 2011 in Frankfurt am Main, Germany. (Photo by Christof Koepsel/Getty Images)

Republican Presidential Candidate Donald Trump kisses his wife Melania Trump as he introduces her during first day of the Republican National Convention in Cleveland, Monday, July 18, 2016. (Photo by Matt Rourke/AP Photo)

Erika Kinsey of Sweden reacts after missing a jump in the women's high jump final during the IAAF World Indoor Athletics Championships in Portland, Oregon March 20, 2016. (Photo by Mike Blake/Reuters)

A model poses at Checkpoint Charlie, a historic place in the German capital to promote the fashion magazine “Sous” on June 12, 2012. (Photo by Maurizio Gambarini)

A participant blows on yellow powder held in her hand during a Holi Party before the Gay Pride Parade in Barcelona on July 9, 2016. (Photo by Lluis Gene/AFP Photo)

Elephant polo players in action during the King's Cup Elephant Polo event 2016 final between teams King Power (blue) and The Elephant Story, in Bangkok, Thailand, March 13, 2016. Team King Power won the annual charity event, which is now in its 14th edition. This year's edition involved 18 elephants brought from tourist trekking camps in Pattaya as well as some unemployed in Surin, and a total of 10 teams encompassing over 40 players, in this event directed at raising funds to improve the lives of elephants and elephant conservation. (Photo by Diego Azubel/EPA)

A man dressed as the Sesame Street character Big Bird sits on a bench waiting to take pictures with people walking through Central Park in New York U.S., November 14, 2016. (Photo by Shannon Stapleton/Reuters)