BOGOTA, May 17 (Reuters) - The governor of a Colombian province handed himself over to prosecutors on Thursday shortly after he was ordered arrested as part of an investigation into ties between politicians and paramilitary death squads.

Hernando Molina is the second governor jailed in the probe that is forcing President Alvaro Uribe to defend his government over charges some of his allies once conspired with militia commanders before they demobilized under a peace deal.

Prosecutors have so far imprisoned 13 lawmakers on charges they colluded with rightist paramilitaries who massacred and controlled swaths of Colombia in the name of countering rebels who are still fighting Latin America's oldest insurgency.

"The presumption of innocence is one guarantee of fundamental rights and so here I am," Molina, governor of northern Cesar province, told reporters as he surrendered at the attorney general's office in Bogota.

A key White House ally, Uribe has received billions in U.S. aid to quell guerrillas from the Revolutionary Armed Forces of Colombia and disarm more than 30,000 paramilitaries who once fought them. Violence has dropped sharply after five years under his government but fighting continues.

The scandal over paramilitary ties is hurting Uribe as seeks to persuade skeptical U.S. Democrats controlling Congress to approve a U.S. free trade pact and clear military and counter-drug aid for the world's No. 1 producer of cocaine.

Critics say the investigation is uncovering collusion between the state and right-wing militias that was an open secret for years. But Uribe backs the probe and says the arrests of his lawmaker allies shows justice is working.

Colombia's paramilitary movement began in the 1980s when wealthy farmers banded together to form vigilante gangs to ward off guerrillas in areas where state presence was weak. But the groups soon began land grabs, massacres and drug trafficking.

Paramilitary commanders are now jailed under a peace deal that grants them short sentences if they give up the gun, renounce crime and confess to atrocities. Thousands were murdered and kidnapped by militia gangs in the 1990s.

Cesar province, on the Caribbean coast and bordering Venezuela, was one of the heartlands of a paramilitary movement led by Rodrigo Tovar, nicknamed "Jorge 40" who is now at the center of the "para-political" scandal.

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