The federal government was so determined to collect the Internet communications of foreign Yahoo customers in 2008 that it threatened the company with fines of $250,000 a day if it did not immediately comply with a secret court order to turn over the data.

The threat — which was made public Thursday as part of about 1,500 pages of previously classified documents that were unsealed by a federal court — adds new details to the public history of a fight that unfolded in secret at the time, as Yahoo challenged the constitutionality of a statute that legalized a form of the Bush administration’s program of warrantless surveillance of foreigners — and lost. Under the Foreign Intelligence Surveillance Act, companies that receive data requests are prohibited by law from talking about the substance of specific requests or even acknowledging they occurred.