The Mexican government has asked the families of two leftist guerrillas missing since 2007 for DNA samples.

The People's Revolutionary Army, or EPR, guerrilla group said in May 2007 that two of its members - Edmundo Reyes Amaya and Gabriel Cruz Sanchez - had disappeared after being arrested in the southern state of Oaxaca.

Months later, the EPR staged attacks on oil pipelines to press for the release of the two men, causing damage to energy infrastructure and millions of dollars in economic losses.

One of the missing men is the brother of Tiburcio Cruz Sanchez, reputedly the leader of the EPR.

On Wednesday, the fourth anniversary of the men's disappearance, the interior ministry and Attorney General's Office issued a joint statement trumpeting progress in the probe and stressing the need for DNA samples to continue the search.

Investigators think the men's disappearance was the work of either organized crime or of rogue officials, according to the statement.

Authorities need DNA samples to compare with those from four unidentified bodies, as well as whatever "substantive information" the families can provide, the ministry and AG's office said.

The EPR, which has traditionally operated only in southern Mexico, burst on the scene with a June 1996 attack on police in the Oaxaca seaside town of Huatulco, a clash that left 13 rebels, cops and civilians dead.

The EPR defines its cause as commemorating and denouncing a June 1995 massacre of 17 Indian peasants in the town of Aguas Blancas, Guerrero.

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