US authorities threatened to fine Yahoo $250,000 a day if it failed to comply with a secret surveillance program requiring it to hand over user data in the name of national security, court documents showed Thursday.

The documents, made public in a rare unsealing by a secretive court panel, "underscore how we had to fight every step of the way to challenge the US government's surveillance efforts," Yahoo general counsel Ron Bell said in a blog post that will again raise privacy concerns.

The documents shed new light on the PRISM snooping program revealed in leaked files from former National Security Agency contractor Edward Snowden.

The program allowed US intelligence services to sweep up massive amounts of data from major Internet firms including Yahoo and Google. Officials have said the deeply contentious program ended in 2011.

The 1,500 pages of documents were ordered released by the Foreign Intelligence Surveillance Court in the case dating from 2007, according to Bell, who said that in 2007, the US government "amended a key law to demand user information from online services."

"We refused to comply with what we viewed as unconstitutional and overbroad surveillance and challenged the US government's authority," he said.

Yahoo's court challenge failed and it was forced to hand over the US user data.

"At one point, the US government threatened the imposition of $250,000 in fines per day if we refused to comply," Bell revealed.

Since the Snowden leaks, Yahoo and others have been seeking to make public these court documents to show they were forced to comply with government requests and made numerous attempts to fight these efforts, rather than simply acquiescing to them, as some critics say.

The opening of these court dockers to the public "is extremely rare," Bell said, adding that the company was in the process of making the 1,500 pages publicly available online.

"We consider this an important win for transparency and hope that these records help promote informed discussion about the relationship between privacy, due process, and intelligence gathering," Bell added.

But he said that "despite the declassification and release, portions of the documents remain sealed and classified to this day, unknown even to our team."

- 'Not reasonable' -

The redacted court records, seen by AFP, showed Yahoo challenged the government on constitutional grounds, saying the surveillance program violated protections against unreasonable search and seizure.

Yahoo said in one brief that the government's requests were "unconstitutional because they permitted warrantless surveillance of US persons' private communications without prior judicial review, and were not reasonable."

The company argued that the program was not merely monitoring overseas targets but some in the United States "with no knowledge that their Internet communications are being retrieved."

Yahoo said the process was "similar to what is done in criminal cases" and would require monitoring from the company's headquarters in Sunnyvale, California.

"The US Supreme Court has never sanctioned warrantless surveillance of US citizens," Yahoo said in another brief.

A document dated May 14, 2008 said Yahoo began complying with the government order two days earlier, on May 12, on "priority user accounts for which the government wanted surveillance."

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