(CNN) -- The killing last weekend of a Catholic priest and two seminary students in southwest Mexico marked the first time that drug cartel hit men have purposefully targeted a clergyman, said Manuel Corral, public relations secretary for Mexico's Council of Bishops.

The Rev. Habacuc Hernandez Benitez, 39, was gunned down as he traveled in a vehicle in the town of Arcelia in the state of Guerrero, together with two seminary students, Eduardo Oregon Benitez, 19, and Silvestre Gonzalez Cambron, 21.

"In this case the drug traffickers followed them and ambushed them," said Corral, who declined to name which cartel is suspected behind the incident.

Authorities have not stated a motive in the case, but Corral said one line of investigation is that one of the seminary students had a family member involved with a drug cartel, and that the three were killed as revenge for some cause.

At least seven priests have been killed in Mexico since 2005 under violent circumstances, including being kidnapped, shot and beaten. Organized crime has been suspected in some of these cases, but Saturday's killing in Guerrero left no doubt of the perpetrators' business, Corral said.

Hernandez and the two seminarians had just celebrated Mass in Arcelia and were leaving the area when a group of unknown gunmen pulled up next to their vehicle and opened fire, according to the local human rights organization Montana Tlachinollan. Local reports added that after shooting the priest's vehicle, the gunmen pulled the men outside and shot them in the back.

The incident paralyzed area residents to the point that no one in the town would go to identify the bodies, Montana Tlachinollan reported. The three victims remained "unknown" until someone from out of town came to officially identify them.

Since taking office in 2006, Mexican President Felipe Calderon has made the fight against drug cartels a priority. He has dispatched more than 25,000 troops to augment local police forces. Last year, drug violence was blamed for the deaths of 78 Mexican soldiers and more than 6,000 others. This year, the drug violence has claimed more than 2,900 lives, according to the newspaper El Universal.

The clergy has not been spared from the spike in drug-related violence that has afflicted regions of Mexico, Corral said.

"The threats are not new," he said. "As drug trafficking continued to grow, from about 2004 on, the priests continued to denounce it, and they themselves were threatened."

In April, the archbishop of the northern state of Durango raised fears of attacks on the clergy after he said that Mexico's most wanted man, Sinaloa cartel boss Joaquin "El Chapo" Guzman, lives in a Durango town and that "everybody knows it except the authorities." The comment, which was made in private conversation, stirred fears of retaliation and the archbishop backed off his statement after it quickly spread.

Of the 15,000 priests in Mexico, Corral estimated that 1,000 had been threatened in some form, and that 300 priests have been threatened directly. Those direct threats -- usually in the form of "shut up, or we'll shut you up" -- have come from intermediaries for organized crime or anonymously, Corral said.

The threats against the clergy have been serious enough that in some 20 cases, priests in Mexico were transferred to other parishes out of regard for their safety, he said.

The targeting of a priest is significant because it marks another boundary crossed by drug cartels, Corral said.

Bishops have noted that in surveys they conduct with families in their dioceses, drug dealing and domestic drug use top the list of problems families face, whereas three or five years ago it didn't make the list. As enforcement action in Mexico and the United States has increased, domestic drug consumption in Mexico has increased.

Domestic drug use has torn families apart and made the new generation of traffickers more violent and less respectful of institutions like the church, Corral said. But these same traffickers who threaten priests are paradoxically the same people that the church wants to reach.

"Rebuilding the social fabric has been a mission of the church," Corral said. "We can't live in fear."

Another notable killing, the 1993 murder of Guadalajara Cardinal Juan Jesus Posadas Ocampo, has been attributed by some as a case of mistaken identity by drug traffickers, though church officials have publicly expressed doubts that the killers were traffickers.

The state of Guerrero, known for its beach destination of Acapulco, in recent years has been the setting of turf wars over drug trafficking routes between the Gulf cartel, the Sinaloa cartel, and an organization known as La Familia.

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