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In the early morning of Feb. 17, 2014, the Mexican drug lord Joaquín Guzmán Loera was in bed with his mistress — one of many — when his personal secretary burst into the room with an urgent message: Troops were at the door. Time to leave.

Mr. Guzmán — naked — ran into the bathroom, beckoning the rest of his household to come with him. Popping the top on his escape hatch, he lifted the lid of the bathtub to reveal a set of wooden stairs leading to a tunnel. As a tactical team of Mexican marines used a battering ram on his front door, the kingpin known as El Chapo disappeared into the tunnel’s humid darkness — and into the annals of criminal myth.

Like other legends that surround Mr. Guzmán, the basic facts of his flight from the marines five years ago have been told so often they have started to develop the haziness of a fable. But on Thursday, the tale was told again, not only with astonishing new details, but by a stunning firsthand source: Lucero Guadalupe Sánchez López, the mistress who escaped into the tunnel at his side.

In an afternoon of testimony at Mr. Guzmán’s drug trial in New York, Ms. Sánchez took the jury through an almost unbelievable drug world love story, starting with the moment she met the crime lord at age 21 and ending with a terrifying trudge through Culiacán’s sewers.