Officials have found three inmates thought to have escaped through a tunnel at a northern Mexico border prison on Monday. The number of still-escaped prisoners now stands at 129.

The public safety secretary of the northern border state of Coahuila says three female inmates were found hiding in a prison visiting area.

Authorities in Coahuila state said the 132 inmates fled the prison in Piedras Negras, a city across the border from Eagle Pass, Texas, through a tunnel that was 21 feet long and 4 feet in diameter, then cut their way through a chain link barrier and escaped onto a neighboring property.

Coahuila Attorney General Homero Ramos Gloria said the director and two other employees of the state prison have been detained for an investigation into the escape and are being questioned about possible involvement by authorities at the penitentiary. The prison houses about 730 inmates and the escape represented almost a fifth of its population.

The tunnel "was not made today. It had been there for months," Ramos told the Milenio TV station. "The prison was not overcrowded, none of our prisons are. We have 132 inmates escaping through a tunnel, and it doesn't make sense."

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Authorities say they also found ropes and electric cables they believe were used in the break.

Federal police units and Mexican troops were deployed to search for the inmates and authorities in Coahuila state offered rewards of up to $15,000 for information leading to the arrests of each prisoner.

Ramos said 70 members of an elite military special forces unit had been sent to search for the prison along with federal police.

The U.S. Customs and Border Protection said it was aware of the prison break and officials are in communication with Mexican law enforcement, according to an e-mailed statement.

Ramos said in a press conference that police are investigating a shootout 160 miles south of Piedras Negras after the prison break to determine if any of the four people killed were fugitives.

He said that 86 of the escaped inmates were serving sentences or pending trials for federal crimes, such as drug trafficking, and the rest faced state charges.

Other Mexican states have said in the past that they are not prepared to handle highly dangerous federal prisoners.

It was one of the larger prison breaks to hit Mexico's troubled penitentiary system in recent years.

In December 2010, 153 inmates escaped from a prison in the northern city of Nuevo Laredo, right across Laredo, Texas. Authorities charged 41 guards with aiding the inmates in that escape. Mexico's drug gangs frequently try to break their members out of prison.

Coahuila, where Monday's prison break took place, has seen a wave of violence tied to the brutal Zetas cartel's battles with the Sinaloa cartel, allies of the now weakened Gulf Cartel.

Authorities in Coahuila did not say which gang was believed to be behind the escape.

Last week, Gulf cartel leader Jorge Eduardo Costilla Sanchez was arrested, leading experts to anticipate an increase in violence in parts of northern Mexico as the Zetas Cartel attempted to take over turf.

In Piedras Negras, family members had gathered outside the prison to hear word of their loved ones.