Six guards were found murdered outside a high-security prison on Mexico's border with Texas yesterday.
Troops sealed off the Matamoros penitentiary and federal agents removed the bodies, some of them blindfolded, from a four-wheel-drive Ford riddled with bullets about half a mile from the prison watchtower.
Interior Minister Santiago Creel said the murders were in retaliation against a government crackdown on prisons, where powerful drug barons often have continued to do business.
"Precisely because we are cleaning the prisons, because we are reviewing law enforcement, because we are advancing against organised crime like no other administration previously, that is why these things are happening," Mr Creel told a news conference.
In recent weeks, Mexican prisons have seen a series of killings and escapes, prompting tighter security, administrative resignations and federal investigations into prison authorities.
Last week, 750 federal soldiers and police took control of the maximum-security prison La Palma outside Mexico City, seeking to head off more violence and a rumored planned break-out attempt following three murders there.
Five high-profile convicts, among them leading drug dealers, were moved from La Palma to other penitentiaries, including the one at Matamoros.
Earlier this week three prisoners escaped from a jail on remote islands off Mexico's Pacific coast.
The government said it would take measures to tighten security across the country's jails, but gave no details.