A federal operation that allowed weapons from the U.S. to pass into the hands of suspected gun smugglers so they could be traced to the higher echelons of Mexican drug cartels has lost track of hundreds of firearms, many of which have been linked to crimes, including the fatal shooting of a Border Patrol agent in December. The investigation, known as Operation Fast and Furious, was conducted even though U.S. authorities suspected that some of the weapons might be used in crimes, according to a variety of federal agents who voiced anguished objections to the operation. Many of the weapons...