The "maximum alert" comes amid what Attorney General Rafael Macedo de la Concha described as a battle for control of the nation's prisons, and temporarily suspends all visits by relatives, allowing inmates to see only their lawyers or government human rights workers.

Investigators reported Friday that the Matamoros prison employees were picked out from among their co-workers by the killers, apparently drug traffickers fighting the latest in a bloody series of turf battles and prison vendettas.

"It was a perfectly planned operation," Macedo de la Concha said. "They (criminal gangs) are trying to undermine the government and its institutions and intimidate public servants ... They have made alliances to try to control the prison."

In a news conference in Matamoros, across from Brownsville, Texas, federal prosecutor Marco Antonio Ramírez said the victims left their jobs early Thursday in three separate cars, among a larger group of employees getting off their 24-hour shift.

But, Ramírez said, a group of assassins, dressed all in black, set up a roadblock on the dirt road leading from the prison, stopping only the cars of the victims and allowing others to pass. It was unclear why the six were targeted.

They included a computer systems technician, two electrical technicians, a guard commander and two drivers.

Ramírez said the killings could have been carried out by the Zetas a group of elite soldiers that were trained to fight drug traffickers but switched sides and now work for the Gulf Cartel.

The killings mirrored another recent attack, also in Tamaulipas state. On Monday, authorities found the bodies of Teodoro Herrera, 67, and two of his adult sons along a road 170 miles (275 kilometers) southwest of Brownsville.

Witnesses said a heavily armed gang dressed in black seized the three men on Saturday along with more than 20 other people, many of them members of the Herrera family. Three remain missing, and the rest were released unharmed.

For months, bodies have turned up in the trunks of cars, abandoned lots and even homes all along the border, evidence of a growing drug war between the Juárez and Gulf cartels, two of Mexico's most powerful drug gangs.

The Gulf Cartel, allegedly headed by Osiel Cárdenas, is based in Tamaulipas state. Federal officials say Cárdenas has continued to run his organization from a prison outside Mexico City after his arrest nearly two years ago in Matamoros and they suggested that he was linked to Thursday's slayings.

The prison employees were kidnapped, handcuffed, blindfolded and then shot to death, Ramírez said. Four showed signs of torture and all were left in a white sport utility vehicle parked across the road from the prison, one of the country's three top federal facilities.

Officials were still searching for the two other cars of the victims as well as two white vehicles believed to be used by the attackers.

Federal officials have described the killings as a direct challenge to President Vicente Fox's crackdown on the drug trade.

Late Thursday, Fox met with members of his Cabinet, including Navy Secretary Marco Antonio Pierrot, Attorney General Rafael Macedo de la Concha, Interior Secretary Santiago Creel, Secretary of the Defense Gerardo Clemente Vega and Public Safety Secretary Ramón Martín Huerta, to discuss the growing violence.

On Friday, Creel told reporters that Cabinet members had decided to declare an "all-out battle" against drug traffickers and would continue to work toward cleaning up the federal prison system.

A special team of federal police was sent in to run La Palma prison, just west of Mexico City, after reports that drug lords were planning escapes and running their cartels from their cells.

At the Matamoros prison, a white, concrete structure surrounded by high walls and barbed wire in the middle of deserted farm fields, soldiers and federal police on Friday sealed off access roads to the facility and refused to let anyone within two kilometers.

Ramírez wouldn't say whether federal officials had taken over operations, but said: "Nothing and no one will intimidate this institution." He added that more federal agents were being sent to Matamoros.

Teresa López the wife of one of the victims, José Guadalupe Medrano Rodríguez, who worked as a driver at the prison was still waiting Friday outside the morgue to recover her husband's remains.

"He never talked about problems or threats," she said.

Relatives of Adolfo Zapata, 29, said they never thought he would be in danger because he didn't have direct contact with the inmates. He just reviewed and maintained the computer network within the prison.

Zapata's father, Francisco, said he wanted officials to find his son's killers.

"I hope God gives me the chance to see their faces," he said.

In yet another prison scandal, the federal Attorney General's Office (PGR) announced Friday that it has arrested six officials accused of permitting a drug trafficker's escape from a Zacatecas state prison on Jan. 18.

It said it has asked the courts to approve charges against the prison's director, deputy director, doctor, security chief and two guards, as well as three alleged accomplices.