A vast trove of about 23,000 unpublished text messages, witness testimony and investigative files obtained by The New York Times point to an answer: Just about every arm of government in that part of southern Mexico had been secretly working for the criminal group for months, putting the machinery of the state in the cartel’s hands and flattening any obstacle that got in its way.

So, when dozens of young men swept into the city of Iguala on passenger buses — not unlike the ones the cartel used to smuggle drugs into the United States — the traffickers mistook their convoy for an intrusion by enemies and gave the order to attack, prosecutors now say.

Reading the cartel’s text messages for the first time last year, in a conference room in the Drug Enforcement Administration’s headquarters in Chicago, Mr. Gómez Trejo realized he had been handed a gold mine.

By that time, the Biden administration had listed Guerreros Unidos among the criminal organizations “that pose the greatest drug threat to the United States,” and much had been written about the cartel’s efforts to corrupt elected officials.

The military knew where at least some of students were being taken, because it was spying on a conversation between a police commander and a cartel boss as they talked about where to deposit the hostages, according to documents made public by the Mexican government.

The group managed to stay alive, thanks in part to some of the drug lords’ wives and one of their mothers, who took over much of the day-to-day business, according to a separate set of hundreds of unpublished exchanges caught on wiretaps.

Saved 92% of original text.