Mexico has been wondering how the powerful drug lord known as El Chapo’s accomplices got their hands on the blueprints to dig a tunnel with such precision. The answer could be quite simple: They may have had them for years.

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MEXICO CITY — Ever since the powerful drug lord known as El Chapo escaped from a maximum-security prison through a mile-long tunnel that opened right into the shower of his cell, Mexico has been wondering how his accomplices got their hands on the blueprints to operate with such pinpoint precision.

The answer could be quite simple: They may have had them for years.

It turns out that the prison is a virtual replica of another lockup that El Chapo, whose real name is Joaquín Guzmán Loera, broke out of in 2001 in an almost equally audacious escape.

In other words, he essentially broke out of the same prison twice.

Authorities believe that for his first escape — by some accounts, he broke out by sneaking out in a laundry cart — Guzmán had the help of a top prison security official who went on to become a trusted member of his Sinaloa cartel.

Investigators think that the confederate, Dámaso López, may have taken a copy of the blueprints for the other prison when he left his job around the time of Guzmán’s earlier escape, a senior Mexican law enforcement official said.

And since the layout of the two prisons is virtually identical, and they were built within a few years of each other, those blueprints could have come in handy when planning this month’s breakout.

The official said that López was now a prime suspect in the hunt for the people who planned and carried out this month’s escape. Beyond the possible blueprints, López is believed to have close knowledge of the layout of the prisons and security procedures. The tunnel makers may have also had the GPS coordinates for Guzmán’s shower stall.

Authorities have so far detained seven prison employees, including four whose job was to watch closed-circuit television monitors that showed Guzmán’s cell and three guards.

But López remains at large.

López was charged with drug trafficking in a 2011 indictment filed in federal court in Virginia, and in 2013 he was described as Guzmán’s right-hand man and a senior lieutenant in the Sinaloa cartel in a statement by the Treasury Department.

The tunnel that Guzmán used to escape included ventilation and lighting and extended for about a mile, under the prison walls, ending in a shaft that opened in a hole about 20 inches by 20 inches in the floor of the tiny shower of his cell.

There was little room for error. Being off by a foot or two would have meant failure.

“Certainly they needed the blueprints,” Interior Secretary Miguel Ángel Osorio Chong said in a news conference last week, although he did not say how Guzmán’s henchmen may have gotten them.

The prison that he escaped from this month, about an hour’s drive from Mexico City, is known as Altiplano, or Federal Social Readaptation Center No. 1, because it was the first of a new set of modern prisons. Construction was completed in 1990, and the center opened in 1991, according to a book by the prison’s first director, Juan Pablo de Tavira.

The prison that Guzmán escaped from in 2001 is Federal Social Readaptation Center No. 2. Located near Guadalajara and commonly known as Puente Grande, it was constructed from 1990 to 1993, according to the book.

A third prison with the same design near Matamoros on the border with Texas was finished in 1994.

The senior Mexican law enforcement official, who spoke on the condition of anonymity because the investigation into Guzmán’s escape is continuing, confirmed that the design for the three prisons was the same.

Satellite images of the prisons show virtually identical layouts. Information available on government websites shows that the prisons were each designed to hold 724 inmates and that the facilities each comprise 300,313 square feet.

After Guzmán’s escape, Mexicans were flabbergasted this week when authorities announced that a computer containing blueprints to another high-security jail, this one in Mexico City, had been stolen. Authorities said the drawings were not the final ones, but they were reported to have moved some prisoners to different cells to protect against possible breakout plans.