WASHINGTON, Aug. 21 (Xinhua) -- The U.S. National Security Agency (NSA) had improperly collected thousands of emails from Americans with no ties to terrorism over three years before a special court ruled it unconstitutional, according to court opinions declassified by intelligence officials on Wednesday.

The release of three secret U.S. court opinions came amid escalating public debates over the intelligence community's surveillance programs, which were first revealed by former defense contractor Edward Snowden in June.

Under the program targeting foreign Internet traffic, the NSA collected as many as 56,000 emails and other communications by Americans starting from 2008, but failed to effectively filter out all the communications between Americans.

The U.S. Foreign Intelligence Surveillance Court was notified in 2011 of the agency's improper collection of emails and other on- line communications.

"For the first time, the government has now advised the court that the volume and nature of the information it has been collecting is fundamentally different from what the court had been led to believe," John D. Bates, a judge on the surveillance court, wrote in his opinion in October, 2011.

The special court ruled the program unconstitutional and the NSA said it had revised its procedures to fix the problem since then.