The small bridge over the Rio Grande links Texas with good times. During spring break every year, thousands of American college students cross from Brownsville to the Mexican town of Matamoros, where a strip of bars with names like Sergeant Pepper's and the London Pub offer hot Mexican food, cold Mexican beer and no American drinking laws. Mark Kilroy, 21, a premed student at the University of Texas, so happily anticipated his March vacation that a friend who went with him recalls, "The whole semester, that was all we talked about." Nobody suspected harm could come to a young man who wanted to do nothing more venturesome than spend a few nights of drinking and fun across the border.

On the first evening, Kilroy and three buddies, all former classmates at Santa Fe (Texas) High, met some girls from Kansas, drank a lot of beer and returned safely to their rooms at the Sheraton Hotel on South Padre Island, Texas, 20 miles away. The second night of beer-drinking revelry went just as well, and at about 2 o'clock in the morning of March 14, the four began walking toward the river, a 15-minute stroll that would take them over the bridge to the U.S. side of the border, where they had parked their car. Two of the men walked ahead, while Kilroy and Bill Huddleston, 21, followed about 20 feet behind. Huddleston paused to step into an alley and do what young men must do after drinking beer all night long. Kilroy waited on the street.

By the time Huddleston came out, Kilroy had vanished. There were no sounds. There were no witnesses. He was just gone.

The first part of Mark Kilroy to be found, four weeks later, was his brain. It turned up in a black cauldron, and it had been boiled in blood over an open fire along with a turtle shell, a horseshoe, a spinal column and other human bones.

His ritual death and dismemberment had been carried out in service to religion—a bizarre, drug-demented occult religion practiced by an American marijuana smuggler operating out of Mexico. Authorities were led to a grave containing Kilroy's body, or at least what remained of it, and after that the uncovering of mutilated corpses went on and on. The first day of digging brought up a dozen bodies, all of them buried on the grounds of Rancho Santa Elena, a grand name for a cattle ranch that is little more than a decrepit corral and a tarpaper-and-wood shanty located just off a twisting, unpaved road. The victims had been slashed, beaten, shot, hanged or boiled alive, the only commonality to their deaths the ritual mutilations that followed. The walls of the 15-foot by 25-foot shack were stained with blood, and scattered about were some items used in the rituals, including a machete and white votive candles in a box bearing a representation of Our Lady of Guadalupe, the patroness of Mexico.

The four men who led police to their cult clubhouse were picked up after one of them, David Serna Valdez, 22, ran a narcotics-interception roadblock near Rancho Santa Elena. Authorities pursued his silver Chevrolet onto the property and found a revolver and a small amount of marijuana in the car. Police showed a picture of the missing American. The ranch caretaker, who has not been charged with any crimes, recognized Kilroy from the photo, and he was also able to give police the names of three other gang members. After 36 hours of questioning—questioning by Mexican authorities can be persuasive—the four men talked.

The gang members told how their leader, Adolfo de Jesus Constanzo, a 26-year-old Cuban-American born and raised in Miami, had killed Kilroy with a single machete blow to the back of his head. Three of the men said they personally had no part in the killings because Constanzo did not permit them to participate. The fourth, Elio Hernandez Rivera, 22, admitted that Constanzo had selected him to be his second-in-command and had marked his body with the symbols denoting fitness to be an occult executioner. On his shoulders, chest and back were scarred arrows, branded into his flesh with a hot knife blade.

Two days later, one of the gang members, Sergio Martinez, 23, was ordered to dig up a 13th body in the presence of the international press. Although he did so in the stench and heat without mask or water, there was no outcry at this display of summary justice. Ultimately, though, there was little satisfaction. Still at large were the leaders: Constanzo, known as El Padrino (the Godfather), and a Mexican woman named Sara Maria Villarreal Aldrete, 24, who led a mysterious double life as the so-called witch of the gang and as an honor student at Texas Southmost College in Brownsville. They were last seen together in Brownsville, in a 1989 Mercedes-Benz. (After they found Aldrete's purse and passport last week, Mexico City police speculated that Constanzo might have killed his partner. But U.S. authorities continued to believe the two were alive and on the lam.)

The four men filled in some details of Kilroy's abduction. They said they were driving a red pickup truck along the main street of Matamoros, on orders from Constanzo "to pick up an Anglo spring-breaker." One of them offered Kilroy a ride, and when he got close, they grabbed him and threw him into the truck. A well-built six-footer, Kilroy struggled free and ran for two blocks, but was recaptured and taken to the shack. At first he was given food and assured he would not be harmed, but less than 12 hours later he was executed. When Kilroy's father, a deeply religious Catholic, learned that some time had passed between abduction and death, he was relieved. "Mark had plenty of time to cry out for God's help," said James Kilroy, 45." When you cry out, God listens."

Santa Fe, where Mark Kilroy lived most of his life, is a simple town: It has a single main street, a single high school and even a singleness of purpose. Like many American towns, it is a notably fine place to grow up.

Jim Kilroy works as a chemical engineer; his wife, Helen, 44, is a volunteer paramedic. They have a second son, Keith, 19. The family regularly attends Mass at Our Lady of Lourdes Catholic Church in the adjoining town of Hitchcock, where about 1,500 mourners turned out to celebrate a Mass of the Resurrection a few days after the body of the church's former altar boy was found. Yellow bows graced the church doors, and yellow ribbons fluttered quietly from trees and signposts; they had been placed there only a few days earlier to remind townspeople to support the search for Mark.

As a high school student, Mark played on the basketball and golf teams, served on the student council and was graduated 14th in a class of 210 in 1986. He first enrolled at Tarleton State University in Stephenville, Texas, on a basketball scholarship but transferred to the University of Texas at Austin after two years, giving up athletics in order to concentrate on premed courses. His grades were 3.0 on a 4-point scale, and he was preparing to take the MCAT. In the emotional days that followed the discovery of the body, Jim Kilroy recalled that when his son was in high school, he would sometimes go to Mark's bedroom to make certain he was studying, only to find him reading the Bible instead. "What do you do?" Kilroy says, smiling at the memory. "He needs to study. But do you go in and tell your son to quit reading the Bible?"

A dolfo de Jesus Constanzo was also raised in a religious environment, but one of a very different character. His last-known address, where he lived with his mother, two brothers and a sister, was in a section of West Miami known as Coral Park Estates. Although the entire family left the block five years ago, many neighbors still fear Constanzo and few are willing to talk about him. "Everyone here is worried [Constanzo] will come back to get them for talking," says one man, standing in the half-opened door of his home. "I've completely protected my house, and if they come by, I'll blow them away."

Constanzo's mother, Delia Aurora Gonzalez del Valle, was married at least three times, although she seems to have raised her family alone. According to neighbors and police records, she was a practitioner of Santería, a religion that evolved when African slaves blended the worship of the gods and spirits of their ancestors with Catholicism.

Santería has as many as 100 million practitioners worldwide, many of them concentrated in the Caribbean and South America. In the United States, the spiritual center is La Iglesia Lukumi Babalu-Aya, a church in Miami. Although rites include animal sacrifice, Santería is a relatively benign religion. But Del Valle practiced it in a peculiarly vindictive manner, said her onetime Miami neighbors, who have accused her of settling grudges by leaving headless animals on doorsteps. Elena Menedez found a dead goose, its head wrapped in a red handkerchief, and Carmen Reigada opened her door to find a decapitated chicken on her stoop shortly after her son quarreled with Del Valle. Constanzo's only known arrest while living in Miami was for shoplifting a chain saw. In 1984 he left to seek his fortune in Mexico, where recently his gang was selling a ton of dope a week and grossing an estimated $200,000.

According to Constanzo's followers, he kept them in line using Santería practices until about nine months ago, when he began immersing himself in the rituals of Palo Mayombe, a malevolent Afro-Caribbean cult that stresses what one expert calls "evil for evil's sake." The effective gods are the all-powerful group leaders, who in an increasing number of cases appear to be drug lords employing the occult as a disciplinary tool. "What we are seeing," says Carl Raschke, a professor of world religions at the University of Denver, "is a religious ideology that is being used almost like a corporate motivational training program to bond and enforce absolute obedience among criminal groups."

One of Constanzo's teaching aids was The Believers, a 1987 Hollywood film starring Martin Sheen and Jimmy Smits that purported to investigate Santería and Palo Mayombe. Constanzo's partner Aldrete was said to use the film to lure men into the circle of the cult and ultimately to the killing shack.

The amalgam of evil created by Constanzo also contained elements of voodoo, Satanism and santismo (a bloody Aztec ritual), but the essential element was human sacrifice. Constanzo told his followers that the spirit and the essence of the victims were absorbed by those who did the sacrificing, and the grisly killings were followed by prayers for strength, riches and protection from the police and physical harm. Not all of Constanzo's victims died for such elevated causes. Some of the 13 murdered men found at the ranch were also in the illegal-drug trade, sacrificed in the name of deals that went bad.

Among the corpses exhumed at the ranch were those of two 14-year-old boys, and when the residences of Aldrete and Constanzo were searched, police came upon disturbing evidence. In Aldrete's Matamoros home, they found an altar, spattered blood and children's clothing. In Constanzo's neat, white, two-story home outside Mexico City, they also discovered baby clothes. Rumors swept through the lower Rio Grande valley shortly afterward that cultists were planning to kidnap children in revenge for the Matamoros discovery, causing panicked parents to pull their children out of schools and officials to post sentries around campuses. The rumors proved completely unfounded. Yet even as Rancho Santa Elena began fading into macabre memory, the horrifying possibility remained that somewhere, more innocents lie, as yet undiscovered in the earth.

—Alan Richman, Bob Stewart in Brownsville, Anne Maier in Santa Fe, Kent Demaret in San Antonio, and Valerie Gladstone in Miami