Residents of nearby villages shook their heads and muttered warnings about this dusty, isolated town that is a three-hour drive south of Mexico City.

"You want to die?" a peasant responded rhetorically when asked for directions. "Many are killed there," commented another.

Ambushers firing shotguns killed four left-wing political militants on a rocky road just outside of town on Sept. 25, the latest in a series of politically inspired murders that have claimed more than 20 lives since 1980 in a population of 15,000.

The case briefly attracted national attention as a fresh example of what Mexico's small but vocal opposition parties claim to be stepped-up violent repression by the ruling Institutional Revolutionary Party, known by its Spanish initials PRI.

Some signs do suggest an effort by local PRI militants to terrorize members of the Marxist Mexican Unified Socialist Party. The Marxist mayor, relatives of the victims and Mexican press reports all have alleged that the presumed killers in the most recent ambush -- members of the Dorantes family -- were allied with a powerful PRI political boss in the nearby city of Tehuacan.

But a stew of small-town rivalries and family feuds also could have contributed to the killings. A senior government official in Mexico made available an official report saying that the Dorantes were indeed the killers, but that they were active in the Marxist party, not the PRI. According to this report, the four Marxist militants had been killed in an internal party squabble.

While it was difficult to sort out the claims, the town's experience showed that rural Mexico, despite the nation's carefully cultivated democratic image, occasionally suffers from the kind of violence and lawlessness often associated with such smaller neighbors to the south as Guatemala and El Salvador.

Police and residents said that the Dorantes family, heavily armed, was hiding out in a house near the edge of town and that nobody dared enter the house unless invited.

"This is something that goes way back. The problem started in the 1930s with the question of the spring," said Reynaldo Fernandez, 40, a brother of two of the most recent victims.

The spring is important because it is the principal source of water for irrigation in this dry plateau flanked by two ridges of the Sierra Madre mountain range. For decades its waters were controlled by 32 well-to-do farmers who reportedly belonged to the PRI, but Marxist militants led an illegal takeover in April 1982 designed to make the water more widely available.

The takeover apparently was popular, as Marxist candidate Raul Correo was elected mayor the next year. But tensions over control of the spring led to the killings of four of the party's militants in May 1981, and to the killings of five PRI activists in November 1982.

Correo said matter-of-factly that "the PRI members were killed in a confrontation" with his party.

The PRI has held a virtual monopoly on political power in Mexico for more than 50 years, but the Marxist Socialists and other small parties of the right and left compete with it. These parties have accused the government of illegally detaining or even killing some of its critics.

Seven left-wing activists, including three deputies of the national legislature, recently staged a 23-day hunger strike to press for information about what they said were more than 500 cases of "disappearances." While the left-wing parties have taken the lead, the principal conservative opposition party, the National Action Party, joined the Marxist Socialists in protesting against the recent violence here.

Jorge Chavez, 44, a Marxist deputy in the legislative assembly for the state of Puebla, in which San Jose Miahuatlan lies, charged that a local PRI boss was the "intellectual author" of the crime 11 days ago.

"They have a policy of selectively eliminating the principal political directors in the region," he said. He charged that 36 Marxist militants have been killed in the state of Puebla in the past 3 1/2 years, but that nobody has ever been arrested for the murders.

The deputy prosecutor for Puebla State, Alfonso Ortiz, did not deny that contention. But he insisted that his office paid no attention to the political affiliation of murder victims.