A controversy about tactics used to break up an extensive Mexican gunrunning ring has prompted federal officials to reevaluate an aggressive law enforcement strategy to stop firearms trafficking. The new scrutiny comes after two separate shootings in the past three months where federal agents were killed and guns recovered by investigators were later traced back to individuals already under investigation by the Bureau of Alcohol, Tobacco, Firearms and Explosives. The top Republican on the Senate Judiciary Committee has charged that ATF agents allowed hundreds of firearms to flow from gun stores in the United States to criminals in Mexico and...