The US documents, released by the National Security Archive at George Washington University which obtained them under the Freedom of Information Act, showed chaos and confusion sweeping across China as authorities crushed the student movement on the night of June 3-4, 1989.
A classified US military report quoted an unnamed source, who observed Tiananmen Square from a hotel room, as calling the crackdown "brutal" with an intention to inflict high casualties.
Troops belonging from the 27th Army, who did not speak the Beijing dialect, "were laughing and shooting at random at any assembly of persons they encountered," the document quoted the source as saying.
The cable also quoted the source as witnessing plainclothes police seizing individuals in a restaurant as part of a "one-by-one crackdown" in Beijing ahead of the main offensive in Tiananmen Square.
But the documents showed that information was sketchy. One US intelligence cable the day after the crackdown quoted a source -- implied to be an official from a foreign government close to China -- as saying that supreme leader Deng Xiaoping had died. In reality, Deng died in 1997.
Another US cable, showing the feeling of crisis spreading through China, said that the military flew foreign currency into the Shanghai branch of the state-run Bank of China as customers raced to withdraw cash following the Tiananmen violence.
Soldiers killed hundreds of unarmed civilians in Tiananmen Square, with some estimates putting the death toll at more than 1,000.
China's government has tried to quash public memories of the crackdown and has stepped up surveillance, censorship and detentions of activists ahead of the 25th anniversary.