An unassuming building housed the tunnel used by the world's most notorious drug lord, Joaquin "El Chapo" Guzman, to carry out a brazen jailbreak on 11 July.
Guzman sparked a massive manhunt after escaping from Altiplano prison in a mile-long underground tunnel that led from a shower into a deserted building, dealing a bitter blow for President Enrique Pena Nieto.
The tunnel was a complex, well-hidden route that came up exactly under the shower in Guzman's cell. It was air conditioned and allowed enough room for a quick exit.
During the escape, Guzman disposed of a bracelet that only he and a few other high-risk inmates had to wear, and smashed bulbs lighting up the tunnel as he fled, authorities have said.
The tunnel would have required noisy digging equipment and produced tons of dirt for disposal.
The construction may have been concealed by a nearby waterway expansion project that started about a year ago near the prison. An open ditch with three reinforced tubes 2.5m wide snaked around the prison would have provided a perfect cover for the building of an escape tunnel.
According to Jose Reveles, a drugs and organised crime expert who penned the acclaimed book on Guzman's 2014 capture, El Chapo: Delivery and Betrayal, the kingpin's Sinaloa Cartel has a lot of experience constructing drug tunnels along Mexico's border with the US.
Mexico has announced a reward of 60m pesos (£2.44m, $3.82m) for information leading to Guzman's capture.