Mexico's three top-security prisons have been placed on maximum alert after the murder of six prison officers.
The bodies were dumped in a vehicle near the prison
The move means all visits to inmates are suspended and staff are being given extra protection.
President Vicente Fox has vowed to wage "the mother of all battles" against drug lords blamed for the killings in the northern Matamoros penitentiary.
The prison workers were kidnapped and blindfolded before being shot and dumped outside the jail on Thursday.
The army has sealed off the prison and the neighbouring city, on the border with Texas.
Mexico's troops last week took over another prison - La Palma near the capital - following a series of killings there.
The latest murders are being seen as the result of an escalating battle between federal authorities and rival drugs gangs, the BBC's Claire Marshall in Mexico City reports.
The prison workers had left the compound early on Thursday in three separate cars after finishing their shifts, federal prosecutor Marco Antonio Ramirez said at Friday's news conference in Matamoros.
He said they were later stopped by gunmen who had set up a roadblock near the prison.
The employees were handcuffed and blindfolded before being shot dead, the prosecutor said.
The bodies were dumped in a bullet-riddled vehicle parked just across the road from the prison. Several of the victims showed signs of torture, Mr Ramirez added.
President Fox linked the attack to a crackdown on drug gangs, apparently operating from behind bars.
"We'll respond with all the might of the state," he said after Thursday's meeting with top security officials.
It is only a week since more than 750 federal soldiers and police were called into take control of another high security prison at La Palma.
This came after a series of murders and rumours that two powerful drugs lords, Osiel Cardenas and Benjamin Arellano Felix, were joining forces inside the prison.
Some inmates had been transferred from La Palma north to Matamoros to try to break up the group, our correspondent says.
Some Mexican security analysts said the Matamoros murders were in revenge for what they described as the "humiliation" dealt to drug lords at La Palma.
Footage of top criminals there - lined up in the prison patio with their heads bowed and hands folded behind their backs - was recently aired on Mexico's TV.