Released this week by the independent Washington-based National Security Archive, the documents showed that US officials were aware of the invasion plans nearly a year in advance.
They adopted a "policy of silence" and even sought to suppress news and discussions on East Timor, including credible reports of Indonesia's massacres of Timorese civilians, according to documents.
East Timor is today an independent nation known as Timor Leste. The population voted in favour of breaking away from Indonesia in a UN-sponsored ballot in August 1999 before gaining full independence in May 2002 after more than two years of UN stewardship.
But the path to independence was bloody. Militia gangs reportedly directed by Indonesia's military went on a killing spree before and after the East Timorese referendum, killing about 1400 independence supporters.
Thirty years after the Indonesian invasion, the formerly secret US documents showed how multiple US administrations tried to conceal information on the territory to avoid a controversy that would prompt a Congressional ban on weapons sales to Indonesia.
The then national security adviser Henry Kissinger told his staff in October 1975: "I'm assuming you're really going to keep your mouth shut on the subject," in response to reports that Indonesia had begun its attack on East Timor.
"These documents also point to the need for genuine international accountability for East Timor's suffering"
Brad Simpson, director of the National Security Archive's Indonesia and East Timor project
The administration of president Gerald Ford knew that Indonesia had invaded East Timor using almost entirely US equipment, and that the use of that equipment for that purpose was illegal, the documents showed.
In 1977, officials of the administration of Ford's successor,
Jimmy Carter, blocked declassification of an explosive cable
transcribing president Ford and by now secretary of state Kissinger's meeting with Indonesian president Suharto.
At the meeting in December 1975, they explicitly approved of the East Timor invasion, according to the documents.
Through the 1980s, US officials continued to receive - and deny or dismiss - credible reports of Indonesia's massacres of Timorese civilians.
The National Security Archive had provided more than 1000 formerly classified US documents to help an East Timorese commission of inquiry into human rights abuses that occurred between 1975 and 1999.
Former US Secretary of State
Henry Kissinger played a key role
East Timor president Xanana Gusmao handed the commission's 2500-page report to the Timorese parliament last Monday but wanted it withheld from the public, amid an outcry from opposition politicians and rights activists.
Brad Simpson, Director of the National Security Archive's Indonesia and East Timor documentation project, said he expected the commission's final report to show that Indonesia's invasion of East Timor and resulting crimes there "occurred in an international context in which the support of powerful nations, especially the United States, was indispensable".
"These documents also point to the need for genuine international accountability for East Timor's suffering," he said.