The US government asked a court to threaten Yahoo with fines of $250,000 (193,500 euros) per day unless it handed over customers' email information to intelligence agencies, court documents released on Thursday showed. The daily fine was to double each week. Yahoo email service is free for customers, but it generates revenue through advertising.

"Our challenge, and a later appeal in the case, did not succeed," Yahoo general counsel Ron Bell said, adding that the court material showed "how we had to fight every step of the way to challenge the US government's surveillance efforts."

The Internet company on Thursday said that it would begin making more of the roughly 1,500 pages of court documents from its failed 2007 legal challenge publicly available.

Bell said that the court upheld a law that is the predecessor to Section 702 of the FISA Amendments Act, more commonly known as the "Prism" program unmasked last year by former National Security Agency (NSA) contractor Edward Snowden.

"Despite the declassification and release, portions of the documents remain sealed and classified to this day, unknown even to our team," Bell said.

Failed Fourth Amendment challenge

The Foreign Intelligence Surveillance Court, whose members are appointed by a Supreme Court justice, has never held a public session. The documents remain heavily redacted, despite their release.

"International terrorists, and [redacted] in particular, use Yahoo to communicate over the Internet," US director of national intelligence at the time, Mike McConnell, said in a court document supporting Washington's stance. "Any further delay in Yahoo's compliance could cause great harm to the United States, as vital foreign intelligence information contained in communications to which only Yahoo has access, will go uncollected."

Yahoo had challenged the demands on the basis of the Fourth Amendment of the US Constitution, guaranteeing that "the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause…"

US Internet companies have been keen to disclose as much information as possible about the secretive procedure leading up to their releasing customers' personal data without public knowledge - in part because of concerns about an impact on their businesses. Earlier this year, Facebook, Microsoft, Yahoo and Google began publishing details on the number of requests for data they receive.

msh/pfd (AFP, AP, Reuters)