PIEDRAS NEGRAS, Mexico (AP) — More than 130 inmates escaped through a tunnel from a prison in northern Mexico on Monday, setting off a massive search by police and soldiers in an area close to the U.S. border.

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."

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.

Earlier on HuffPost:

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  • May 13, 2012

    Suspected drug gang hitmen dumped 49 mutilated bodies, stuffed in bags, on a highway outside the northern industrial city of Monterrey. <em>Caption: Federal police stand guard on a vehicle behind a forensic truck containing bodies found on the highway connecting the northern Mexican metropolis of Monterrey to the U.S. border, along the Reynosa-Cadereyta road, in the town of San Juan near the city of Monterrey, Mexico, Sunday, May 13, 2012. (AP Photo/Christian Palma)</em>

  • May 4, 2012

    The bodies of nine people were found hanging from a bridge and 14 others found dismembered in the city of Nuevo Laredo, just across the U.S. border from Laredo, in Texas. <em>Caption: Four of nine corpses are seen hanging from a bridge in the Mexican border city of Nuevo Laredo, Tamaulipas state, early morning on May 4, 2012. (RAUL LLAMAS/AFP/GettyImages)</em>

  • Feb 19, 2012

    A fight between rival gangs at a prison just outside Monterrey in northern Mexico leaves 44 dead. <em>Caption: Araceli Guevara Ontiveros, the sister of Francisco Guevara Ontiveros --one of the 44 dead in a riot at The Apodaca prison-- is comforted while crying on his coffin during his wake in Monterrey, Nuevo Leon state, on February 21, 2012. (Julio Cesar Aguilar/AFP/Getty Images)</em>

  • Nov 24, 2011

    More than 20 bodies are found in cars in Mexico's second city, Guadalajara, a day after the burned bodies of 16 people are found in the home state of the country's powerful drug lord, Joaquin "Shorty" Guzman. <em>Caption: A member of the forensic service carries one of the 26 corpses found this morning in three vehicles abandoned in Mexico's second most populous city of Guadalajara, Mexico, on November 24, 2011. (HECTOR GUERRERO/AFP/Getty Images)</em>

  • Oct 6, 2011

    Mexican security forces find 32 bodies at several locations around Veracruz, just two days after the government unveiled a plan to bolster security in Veracruz state. <em>Caption: Mexican army soldiers walk towards their vehicle after seven bodies were found inside a vehicle in the Gulf port city of Veracruz, Mexico, late Friday Oct. 7, 2011. (AP Photo/Felix Marquez)</em>

  • Sept 20, 2011

    Thirty-five bodies are found abandoned in two trucks on an underpass in the eastern Gulf city of Veracruz, which had been largely untouched by the violence. <em>Caption: Mexican marines stand guard in streets of Veracruz State, Mexico on 24 January 2012. (JOHAN ORDONEZ/AFP/Getty Images)</em>

  • Aug 25, 2011

    Masked gunmen torch a casino in Monterrey, killing 52 people, most of them women. The attack takes less than three minutes. <em>Caption: Relatives of victims cry in front of the Casino Royale, in Monterrey, Mexico, on August 27, 2011. (Ronaldo Schemidt/AFP/Getty Images)</em>

  • Aug 20, 2011

    Five headless bodies were found in Acapulco, taking the number of people killed in the popular Pacific resort to at least 25 in that one week. <em>Caption: Forensic personnel move the corpse of a person murdered in a hotel at the La Guinea neighbordhood in the town of Acapulco, Guerrero state, Mexico on August 24, 2011. (Pedro PARDO/AFP/Getty Images)</em>

  • April 2011

    Officials unearthed the first of what turned out to be more than 450 bodies buried in mass graves in the northern states of Durango and Tamaulipas. <em>Caption: Forensic personnel unload at the morgue bodies of people killed execution-style in Matamoros, Tamaulipas State, Mexico, on April, 11, 2011. (STR/AFP/Getty Images)</em>

  • Aug 25, 2010

    Marines found the bodies of 58 men and 14 women at a ranch near the Gulf of Mexico in Tamaulipas state, 90 miles from the Texas border, after a firefight with drug hitmen in which three gunmen and a marine died. <em>Caption: A worker wearing protective suit and boots walks between two refrigerated trucks parked outside a funeral home where the bodies of 72 men and women that were allegedly killed by the Zetas drug gang, are kept in San Fernando, just 100 miles from the the Mexican border with the U.S. near the city of Matamoros, Thursday Aug. 26, 2010. (AP Photo/Guillermo Arias)</em>

  • July 24, 2010

    Police unearthed 51 bodies in a grave outside Mexico's business capital, Monterrey, in northern Mexico over several days. Some corpses were burned beyond recognition. <em>Caption: View of the remains of two burnt bodies found in Monterrey, Nuevo Leon state, on October 20, 2010. The bodies were found under burnt wooden pallets. (Dario Leon/AFP/Getty Images)</em>

  • July 18, 2010

    Gunmen burst into a birthday party in the northern city of Torreon, using automatic weapons to kill 17 party-goers and wound 18 others. Mexican authorities said later those responsible were incarcerated cartel hitmen let out of jail by corrupt officials. The killers allegedly borrowed weapons and vehicles from prison guards and later returned to their cells. <em>Caption: Police officers patrol a street in Torreon, in the Mexican northern state of Coahuila, Monday, July 19, 2010. (AP Photo/Ramon Sotomayor)</em>

  • June 28, 2010

    Suspected cartel gunmen shot and killed a popular gubernatorial candidate in the northern state of Tamaulipas in the worst cartel attack on a politician to date. Rodolfo Torre, 46, and four aides from the opposition Institutional Revolutionary Party, or PRI, were ambushed on their way to a campaign event for the July 4 state election. <em>Caption: A billboard with the portrait of the candidate for Governor of Tamaulipas state for the Institutional Revolutionary Party (PRI), Rodolfo Torre, and reading 'A leader forever' is seen during his funeral at the Polyforum in Ciudad Victoria, Tamaulipas state, Mexico, on June 29, 2010. (LUIS ACOSTA/AFP/Getty Images)</em>

  • March 13, 2010

    Hitmen killed three people linked to the U.S. Consulate in Ciudad Juarez in March, provoking "outrage" from U.S. President Barack Obama. <em>Caption:The U.S. national flag flutters at half-mast at the entrance of the consulate of the United States in Ciudad Juarez, Chihuahua state, Mexico on March 15, 2010. (Jesus Alcazar/AFP/Getty Images)</em>

  • Jan 31, 2010

    Suspected cartel assailants killed 13 high school students and two adults at a party in Ciudad Juarez across from El Paso, Texas. <em>Caption: Students enter a high school in Ciudad Juarez, a city ridden by homicidal violence along Mexico's border with Texas. (Tim Johnson/MCT via Getty Images)</em>

  • Sept 15, 2008

    Suspected members of the Zetas drug gang tossed grenades into a crowd celebrating Mexico's independence day in the western city of Morelia, killing eight people and wounding more than 100. <em>Caption: From left to right: Julio Cesar Mondragon Mendoza, Juan Carlos Castro Galeana, and Alfredo Rosas Elicea, are members of a group of hitmen called the 'Zetas' shown to the press at the General Attorney's office in Mexico City, on September 26, 2008. The three gangsters confessed that they are the authors of a grenade attack that killed eight people during the celebration of Independence Day in Morelia, western Mexico. (STR/AFP/Getty Images)</em>