A group of 33 Democrats in the U.S. House of Representatives urged Secretary of State Hillary Clinton to pressure Mexico over the plight of Central American migrants and make respect for their human rights a key element of U.S. bilateral assistance.

“We believe that strengthening Mexico’s efforts to evaluate performance and increase the accountability of its security forces, including the Federal Police and the INM (National Migration Institute) should be key elements of U.S. assistance to Mexico,” the group of legislators headed by Raul Grijalva (D-Ariz.), co-chair of the Congressional Progressive Caucus, said in a letter sent Friday to Clinton.

That is the only way to “ensure that crimes and human rights violations committed by members of these federal agencies do not go unpunished.”