WASHINGTON — A US Justice Department employee considered a key figure in a botched US program to track guns into Mexico has refused to testify to Congress, claiming a constitutional right.

Patrick Cunningham, head of the criminal division of the Arizona state Attorney General's office, said he would invoke his Fifth Amendment rights against self-incrimination, according to a letter to Congress from his lawyer, revealed Friday.

"I am writing to advise you that my client is going to assert his constitutional privilege not to be compelled to be a witness against himself," lawyer Tobin Romero wrote to Republican congressman Darrell Issa, chairman of the House Oversight and Government Reform Committee.

On Wednesday, Issa subpoenaed Cunningham to testify no later than next Tuesday after several failed attempts to question him.

According to Republican lawmakers, Cunningham played a key role in approving the tactics of the 'Fast and Furious' program, which allowed people linked to drug gangs to buy automatic weapons in the United States in order to track their trafficking routes.

Cunningham's lawyer said they had "blamed him unfairly" and underlined that his client took his post in Arizona in 2010, when the operation already was under way.

Issa said in a statement it was the first time a government official had refused to testify in the probe.

"The assertion of the Fifth Amendment by a senior justice official is a significant indictment of the (Justice) Department's integrity in Operation Fast and Furious," he wrote.

He said it "raises questions" about whether President Barack Obama and Attorney General Eric Holder knew about the operation, a charge they have denied.

The sting, which ran between November 2009 and the end of 2010, observed some 2,000 weapons pass into Mexico, losing trace of many of them.

It has caused an uproar in Mexico, where rising drug-related violence has left some 50,000 dead in the past five years.

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