By Frank Jack Daniel

MEXICO CITY, Sept 13 (Reuters) - The leftist rebels behind huge pipeline bombings in Mexico this week are from a small guerrilla group held together by family ties that has long personal and political grudges against the government.

The Popular Revolutionary Army, or EPR, blew up gas and oil pipelines on Monday in their biggest attack on economic targets since emerging in mountain villages of southern Mexico in the mid-1990s to kill dozens of police.

Tiburcio Cruz Sanchez, known as "The Professor," is the man the army says heads the EPR. He comes from a family of guerrillas from the southern state of Oaxaca that has been active since the 1970s.

Two of his sons are in jail for bombing banks. Human rights activists say they are innocent but were arrested to hit back at the elusive Professor and his wife, who is from another small rebel dynasty.

The EPR, believed to number under 1,000 members, launched a campaign of economic sabotage in July with bomb attacks on energy installations, repeated on a bigger scale this week.

The latest blasts caused hundreds of millions of dollars of damage to state oil firm Pemex and thousands of businesses.

The Marxist guerrillas' main direct demand is for the government to give up two rebels it says were taken by security forces from the streets of Oaxaca city in May.

One of them, Gabriel Cruz, is The Professor's brother and had lived in hiding and under false names for 25 years.

"These men are key players and they know important information about the whole movement," said veteran reporter and guerrilla expert Jose Gil Olmos.

The government denies taking the two men, and says they were perhaps killed in a feud between rebel leaders, many of whom come from three overlapping families who have led guerrilla groups in southern Mexico for decades.

The disappearance of the pair touched a sensitive spot for the guerrillas and brought back memories of Mexico's so-called dirty war in the 1970s, when the army 'disappeared' hundreds of people accused of being linked to rebels.

CORN AND SICKLE

Mexico was shocked by the scale of this week's attacks. They cut natural gas supplies to industry and halted output at most of Mexico's steel plants and companies like Volkswagen.

The EPR, which calls for land reform and ultimately a socialist state, had kept a low profile for years after in-fighting and an army clampdown left the group in disarray.

"They are hitting the system where it hurts," said Carlos Mendoza, who made a film about the group. "They are sending a message that they have more capacity than has been attributed to them for a long time."

On its website (http://www.pdpr-epr.org/), which shows a Soviet-style sickle crossed with rural Mexican symbols of an ear of corn and a machete, the EPR outlines a goal of a socialist-style command economy.

Some experts say the power of recent bombings may be a sign they have new leaders or bombmakers.

"You might have a new radical, a more violent person that has stepped to the forefront," said Fred Burton, a former U.S. counter-terrorism agent who works for security consultants Stratfor.

The EPR is a smaller but more aggressive group than Mexico's most famous rebels, the Zapatistas, who control territory in the southern state of Chiapas but have mostly shunned violence since they briefly took over towns in 1994.

Deep poverty in rural regions and a breakdown in government intelligence gathering since Mexico ended seven decades of one-party rule in 2000 have allowed the EPR to regroup and possibly infiltrate institutions like Pemex.

"The people involved are well prepared," said Gil Olmos.

Since the July attacks, Mexico has deployed more army and police to guard its vast network of pipelines, which stretches for more than 8,700 miles (14,000 km), but President Felipe Calderon warns it is impossible to completely secure.

Calderon launched a major offensive against powerful drug cartels when he took office last year. With intelligence agents limited, the bombings will tax already stretched resources.

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