President Diaz gave them orders ahead of time to shoot, and the Chief of Staff of the Military had snipers in the buildings with orders to open fire once people ran. Meanwhile it took like a day to even bring in armed soldiers at Tiananmen even after MONTHS of protesting.

From "Our Man in the CIA" by Morley page 269-270

known as the Olympic Battalion, had their own instructions. They were to wear civilian clothes with a white glove on the left hand and post themselves in the doorways of the Chihuahua building. When they got the signal, in the form of a flare, they were to prevent the entrance or exit of anyone to the plaza while the student leaders were being detained. Finally, a group of police officers got orders to arrest the leaders of the national strike council.

What virtually no one knew until more than thirty years later was that Luis Gutierrez Oropeza, the chief of staff of the Mexican military, had posted ten men with guns on the upper floor of the Chihuahua building and given them orders to shoot into the crowd. He was acting on orders of Diaz Ordaz, according to a revelatory account published in 1999 in the news¬ weekly Proceso, by Julio Scherer and Carlos Mosivais, two of the country's best-known journalists. Oropeza was the link between Diaz Ordaz and Echeverria, according to Mario Moya Palencia, a Mexican political insider at the time. Oropeza was also a friend and occasional dinner guest of Win Scott's. He might have been a LITEMPO agent. There is no evidence that he acted on the CIA’s behest in October 1968.

A wave of people ran to the far end of the plaza, only to meet a line of oncoming soldiers. They ran the other way — into the free-fire zone. It was a “closed circle of hell,” said historian Krauze: a “terror operation.” The shooting went on for an hour and then began to diminish. It started to rain. The tanks opened fire. “The hail of bullets fired at the Chihuahua building became so intense that around seven p.m. a large section of the building caught on fire,” wrote reporter Jorge Aviles of El Universal. ‘All the floors from the tenth to the thirteenth floors were enveloped in flames and many families were forced to leave the unit, amid the heavy gunfire, carrying the children in their arms and risking their lives.” Inside the floors of the hallways were sticky with blood. The shooting continued until eleven o’clock that night. Five thousand soldiers fired a total of 15,000 rounds. Two thousand people were arrested, many of them stripped, beaten, and abused. Lights were extinguished, telephone service was cut off, photographers were forbid¬ den from taking pictures, and even ambulances were turned away

Diaz also was not so much on the payroll as he was literally besties with Winston Scott, the CIA chief in Mexico.

Scott's agents were identified in CIA files by specific numbers. LITEMPO-1 was Emilio Bolanos, a nephew of Gustavo Díaz Ordaz, Minister of Gobernación and then President in the 1960s. Díaz Ordaz was LITEMPO-2. Like his predecessor Adolfo Lopez Mateos, he was a personal friend of Scott's. Both men attended Scott's wedding to his third wife in December 1962, with Lopez Mateos standing in as padrino, or chief witness, to the ceremony.

We know of 4 consecutive Mexican presidents confirmed CIAasset status, and at least 1 explicitly being CIA before becoming president.