Plaintiffs in a class action lawsuit challenging the legality of the National Security Agency's surveillance program are heading to court Thursday in an attempt to unseal evidence.

The federal government has maintained that its once-secret surveillance program is legal and targets only communications with suspected terrorist links. AT&T has argued that it has lawfully cooperated with legitimate government investigations and cannot comment on its possible role in the investigations. Both parties argue that the lawsuit could jeopardize national security.

The Electronic Frontier Foundation and other plaintiffs accuse AT&T of working with the NSA to illegally spy on millions of innocent Americans by eavesdropping without warrants.

Media groups are trying to convince a judge in San Francisco to unseal a declaration from retired telecommunications technician Mark Klein. Klein is a former AT&T employee who said he helped set up a system in which government investigators could received duplicates of AT&T's customers' communications. The media groups also want U.S. District Court Judge Vaughn Walker to unseal internal AT&T documents and parts of a statement from an EFF expert witness.

Some of the evidence was already released in redacted form. News media reported the warrantless surveillance program. EFF and other critics have characterized the program as a massive data-mining initiative that scans ordinary Americans' telephone and Internet communications for suspicious names, numbers, words, or patterns of communication.

The EFF accuses AT&T of assisting the government and violating its customers' privacy rights by giving it "unfettered access to its Daytona database, containing more than 300 terabytes of caller information.