Mexican authorities hunted for 131 escaped inmates near the US border after a mass prison break which officials suspect was organized by the ultra-violent Zetas drug cartel.
Some 5,000 Mexican soldiers and police fanned out across the Texas border region, causing traffic jams as they inspected trucks and cars, one day after the prisoners fled through a tunnel in the northern state of Coahuila.
Across the Rio Grande, US authorities were on alert and patrolled their side of the border with helicopters amid concerns that the inmates could make a run for the United States.
Jorge Luis Moran, public security secretary in the northern state of Coahuila, told reporters that those at large had links to the Zetas.
"The line of investigation is that the Zetas cartel was able to organize the escape because the prisoners who were held on federal charges had ties with this group," Moran told local radio.
State police officers were attacked by gunmen with high-caliber weapons and grenades when they tried to reach the prison after Monday's escape, raising suspicions that the Zetas were involved, he added.
A source close to the state prosecutor's office said the escape may be linked to the Zetas' battle against the rival Gulf Cartel.
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.
Last February, 30 Zetas members escaped from a prison in the northern state of Nuevo Leon during a massacre that killed 44 inmates from the Gulf Cartel. Some prison guards confessed to taking part in the plot.
And in December 2010, 141 inmates fled from the Nuevo Laredo prison in the northeastern state of Tamaulipas.
Mexican President Felipe Calderon called the latest escape "deplorable" and slammed the "vulnerability" of state penitentiaries, noting that more than 1,000 inmates have escaped from such prisons in the last six years.
Monday's escape took place in the border city of Piedras Negras. Officials revised the number of escaped inmates from 132 to 131.
The prisoners escaped one-by-one through a seven-meter-long (22-foot-long) tunnel that started in the carpentry workshop and surfaced at the prison's northern watchtower.
After emerging from the tunnel, which was 2.90 meters (9.5 feet) deep and 1.20 meters wide, the inmates cut through a perimeter fence and slipped away.
The prison's director, security chief and shift guard have been detained and all of the penitentiary's guards will be summoned for questioning over possible complicity in the escape, said state attorney general Homero Ramos Gloria.
The guards must explain "why nobody found out that this tunnel was being built," he added.
Authorities in Coahuila said they were investigating whether escaped convicts were involved in a shoot-out with a special police unit in the nearby town of Castanos in which four suspects were killed.
The prosecutor's office said 86 of the escaped inmates were in prison for federal crimes while the others faced a variety of charges.
The state government offered rewards of 200,000 pesos ($15,600) for information leading to the capture of each inmate.