A gunman opened fire at Mexico's Teotihuacán pyramids on Monday, killing a Canadian woman and injuring 13 people, including six Americans, in what authorities described as a carefully planned attack. Julio César Jasso Ramírez, 27, a Mexican citizen from Guerrero state, climbed the Pyramid of the Moon and began shooting into a crowd of tourists before engaging in a standoff with security forces.

In the attacker's backpack, authorities discovered literature, photos and notes "allegedly related to violent events that are believed to have occurred in the United States in April of 1999," according to State Attorney General José Luis Cervantes Martínez. The materials may reference the Columbine High School massacre, which occurred on April 20, 1999—exactly 27 years before the Teotihuacán attack. Investigators found more than 50 unfired rounds in his bag along with tactical equipment including knives, gloves and goggles.

Jasso fired down at Mexican National Guardsmen who responded to emergency calls, eventually climbing the pyramid's steep steps. A guardsman shot him in the leg, immobilizing him, after which he used his .38-caliber revolver to take his own life. President Claudia Sheinbaum stated the shooting was "not linked to organized crime, but rather the act of an individual" influenced by events beyond Mexico. The violence at the site, which drew 1.8 million visitors last year, is believed to be the first at a major Mexican archaeological site in modern history.