By Hugh Bronstein

BOGOTA, July 11 (Reuters) - A scandal linking political allies of Colombian President Alvaro Uribe to right-wing death squads deepened on Wednesday when the Supreme Court opened an investigation into his first cousin, Senator Mario Uribe.

The president's international standing has already been damaged by the scandal in which his former security chief and some of his closest allies in Congress have been jailed and are awaiting trial for supporting paramilitary militias.

"The Supreme Court opened a preliminary investigation against senators Mario Uribe, Zulema Jattin and Julio Manzur for possible links to illegal armed groups," said a spokesman for the court.

A presidential spokesman had no immediate comment.

The scandal began late last year when members of Congress admitted they had signed a document agreeing to support paramilitary groups formed in the 1980s to help defend drug lords and cattle ranchers against left-wing rebels.

Since the 1990s the "paras" have grown rich on Colombia's multibillion-dollar cocaine trade and notorious for massacring peasants suspected of leftist sympathies.

"It is very worrying that the 'para-political' scandal is getting closer and closer to the president's inner circle," said Jorge Rojas, head of Colombia's top human rights group CODHES.

Colombia has received billions of dollars in U.S. aid to crack down on the drug trade. Democrats in control of the U.S. Congress are toughening conditions on that aid and are putting off voting on a free trade pact with Colombia due to concerns about human rights in the Andean country.

More than 31,000 paramilitaries have turned in their guns over the last three years in a deal with the government offering them benefits, including reduced jail terms, for crimes like murder, torture and drug smuggling.

But human rights groups say paramilitary leaders have not dismantled their criminal structures and the government admits that thousands of demobilized militia fighters have regrouped into new crime gangs. (Additional reporting by Monica Garcia)

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