A brutal Mexican drug cartel is thought to have plotted a mass prison escape in order to replenish its weakened ranks.
Los Zetas is believed to have orchestrated the mass tunnel escape of 131 inmates from a prison in Piedras Negras - a city across the border from Eagle Pass in Texas.
The escape tunnel was 21ft (6.5m) long and 4ft (1.2m) in diameter, and after passing through it, the prisoners cut their way through a chain link barrier.
Officials said the jailbreak was possibly spearheaded to revive the gang, that has suffered at the hands of a rival cartel.
Some 5,000 Mexican soldiers and police have fanned out across the Texas border region as they inspect trucks and cars.
US authorities are patrolling their side of the border with helicopters amid concerns that the inmates could make a run for the US, although so far no escapees had been reported crossing the border.
Jorge Luis Moran, the public security secretary in the northern state of Coahuila where the prison is located, told reporters: "Clearly, the Zetas are behind this escape.
"The line of investigation is that the Zetas cartel was able to organise the escape because the prisoners who were held on federal charges had ties with this group."
Some prisoners who are not in the gang were also forced to go along, said Mr Moran.
Police are investigating whether the prison break might be linked to the seizure of empty passenger buses in the region that could have been used to pick up the escapees.
They are also probing if the escape was connected with a shoot-out with a special police unit in the nearby town of Castanos in which four suspects were killed.
Coahuila Attorney General Homero Ramos said 86 of the escaped inmates were serving sentences or awaiting verdicts for federal crimes, such as drug trafficking, and the rest faced state charges.
The Zetas cartel has been fighting a bloody turf battle with the Sinaloa cartel in the border state.
Mr Moran said Los Zetas controlled the drug corridor until 2010, when members of the powerful Sinaloa gang were sent to the state. The Sinaloa cartel is led by Mexico's most wanted man, Joaquin Guzman.
Mr Moran said Los Zetas has also been hit by arrests, fatal shootings and guns seizures, adding: "They are running out of people."
Several prison escapes have taken place in the last two years in Mexico, a country struggling to stem a relentless wave of murders and kidnappings committed by an array of warring traffickers.
"The drug cartels have taken their internal wars into the prisons," said Jose Luis Musi, a prison issues expert at the United Nations University.
