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)

Men are held by Iraqi national security agents, to be interrogated at a checkpoint, as oil fields burn in Qayara, south of Mosul, Iraq, Saturday, November 5, 2016. Islamic State fighters launch counterattacks in the thin strip of territory Iraqi special forces have recaptured in eastern Mosul, highlighting the challenges ahead as the battle moves into more densely populated neighborhoods where coalition air power must be used more selectively. (Photo by Felipe Dana/AP Photo)

In this third of seven sequential photos, Elian Gonzalez is held in a closet by Donato Dalrymple, one of the two men who rescued the boy from the ocean, right, as government officials search the home of Lazaro Gonzalez for the young boy, early morning, April 22, 2000, in Miami, Florida. Armed federal agents seized Elian Gonzalez from the home of his Miami relatives before dawn, firing tear gas into an angry crowd as they left the scene with the weeping 6-year-old boy. This photo won the Prize in 2001. (Photo by Alan Diaz/AP Photo)

Police officers detain a Navalny supporter during a protest in St. Petersburg, Russia, Tuesday, February 2, 2021. A Moscow court has ordered Russian opposition leader Alexei Navalny to prison for more than 2 1/2 years on charges that he violated the terms of his probation while he was recuperating in Germany from nerve-agent poisoning. Navalny, who is the most prominent critic of President Vladimir Putin, had earlier denounced the proceedings as a vain attempt by the Kremlin to scare millions of Russians into submission. (Photo by Dmitri Lovetsky/AP Photo)

In this aerial view Haitian migrants queue to receive food at a shelter in Ciudad Acuna, Coahuila State, Mexico, on September 23, 2021. At least 50 police vehicles carrying more than a hundred agents are blocking the border crossing at the river that separates the Mexican city of Ciudad Acuña from the United States, AFP reported. (Photo by Pedro Pardo/AFP Photo)

A model walks the runway at the Christian Siriano X Lane Bryant Collection at United Nations on May 9, 2016 in New York City. (Photo by JP Yim/Getty Images)

Julien MacDonald and Izabel Goulart backstage at Julien Macdonald presents Julien x Gabriela show during Spring Summer 2020 London Fashion Week in London, United Kingdom on September 16, 2019. (Photo by Backgrid USA)

Lovot companion robots by Groove X wander in front their booth at CES Unveiled during the 2020 CES in Las Vegas, Nevada, January 5, 2020. (Photo by Steve Marcus/Reuters)