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."

The United States has pledged some $1.6 billion to Mexico through the Merida Initiative, which was launched in 2008 to combat drug trafficking and organized crime in Mexico and Central America.

The State Department said Friday it has already disbursed $700 million of that total, with the figure set to rise to $900 million by year's end.

Through that initiative, the United States has provided significant support to Mexico's Federal Police and the INM, the lawmakers said, but "regretfully, agents from these same agencies have also been implicated in multiple cases of abuse of migrants."

They recalled that, according to a report released in February by Mexico's independent National Human Rights Commission, or CNDH, 11,333 migrants were kidnapping victims between April and September 2010.

Apart from the abductions, migrants heading north "are also frequently subjected to other abuses during their journey such as torture, extortion, and robbery; approximately six in 10 migrant women are raped," the letter said.

"In investigations conducted by the CNDH, they found that nearly 9 percent of migrants who were victims signaled the collusion or direct participation of agents from the INM or federal, state, or local police."

The letter also referred to the 2010 massacre in the northern state of Tamaulipas of 72 mostly Central American illegal migrants - apparently after they refused to work for a drug cartel - and said it put the spotlight on the perilous journey migrants face, although the lawmakers said that "since then little has been done to effectively address this humanitarian crisis."

The lawmakers said that "while the United States immigration debate focuses on the status of migrants already in this country, little has been said about what happens to migrants before they arrive."

"Traveling from Mexico or Central America to the United States is one of the most dangerous journeys in the world, and letting the violence and abuse continue isn't in anyone's interest."

They therefore called on Clinton to pressure the Mexican government to take steps to protect the safety of migrants traversing Mexican territory, as well as of activists seeking to shield them from abuse.

"We also encourage the U.S. Department of State to discuss with our Mexican counterparts ways U.S. assistance could be directed to provide additional humanitarian assistance" to migrants, the letter said.

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