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But Mr. Holder said the U.S. and Mexico must continue to work together to stop the flow of drugs and the violence the trade fuels, much of which is believed to be perpetrated with high-powered weapons smuggled to Mexico’s streets from the U.S.

“As President Obama indicated during the campaign, there are just a few gun-related changes that we would like to make, and among them would be to reinstitute the ban on the sale of assault weapons,” Mr. Holder said. “I think that will have a positive impact in Mexico, at a minimum.”

Thomas Schweich, a lawyer with the international firm Bryan Cave who is special diplomatic representative for Latin America to the United Nations Office on Drugs and Crime, finds reason to be hopeful.

“It’s a message to the cartels that the U.S. and Mexico are working ever so closely together,” he said. “This is just the beginning of this effort to crack down on these cartels. It’s not the end.”