“It was the only way to dismantle an entire firearms-trafficking ring and stop the thousands of guns flowing to Mexico,” said William D. Newell, a veteran federal agent who spent five years as the head of the Bureau of Alcohol, Tobacco, Firearms and Explosives in Phoenix.

In his first public interview about the operation, Newell said he believed that he and his agents were working the largest gun-trafficking case of their careers and finally had a window into Mexico’s powerful Sinaloa cartel. To identify cartel members, ATF agents, beginning in 2009, watched as about 2,000 weapons purchased at Phoenix gun stores hit the streets; their goal was to trace them to the cartel…

The plan was permitted under ATF rules, had the legal backing of U.S. Attorney Dennis K. Burke in Phoenix, was approved by Newell’s ATF superiors and was funded by a regional task force of the Justice Department, ATF’s parent agency.

Newell scoffs when he hears lawmakers and others call the tactics “controversial.” Three similar operations had been tried during the George W. Bush administration by the same ATF field office, including a Tucson plan dubbed Operation Wide Receiver that involved many fewer guns, about 300.