The Obama administration has filed what appears to be the first legal defense of the National Security Administration's dragnet of American phone data since Edward Snowden first leaked details to the public.

As part of a legal proceedings set for this Thursday, the US Department of Justice on Friday sent a letter to Judge William Pauley in the US District Court in Manhattan, outlining the broad contours of the "highly sensitive and, in many respects, still classified intelligence-collection program."

The letter revealed some of the strategies and arguments the Obama administration will employ as it defends the NSA program against the American Civil Liberties Union (ACLU), which filed a lawsuit in June.

With a number of other lawsuits challenging the secret collection of data, the filing also represents one of the first salvos within the new legal and political landscape created by the Snowden leaks. The battles ahead in Congress and in courtrooms will most likely continue for years, and they will help redefine the parameters of privacy in a digital age still dominated by fears of terrorism and threats to national security.

In its lawsuit, the ACLU is asking the court to halt the program immediately with a preliminary injunction before the case is decided. The suit argues in part that the government's clandestine data mining violates First Amendment rights of free speech and association – that it, in effect, puts a "chill" on free speech – and that it violates the Fourth Amendment's protections against warrantless search and seizure.

The Justice Department disagrees with those assessments. The letter, by David Jones, an assistant US attorney, argues that the program's checks and balances are adequate. For example, the government may not eavesdrop on anyone's phone calls or record anything participants say. All it can do is collect phone numbers making and receiving certain calls, as well as the date, time, and duration of each call – the so called "metadata."

Even then, the letter continues, “the Government is prohibited ... from indiscriminately sifting through the data. The data-base may only be queried for intelligence purposes by NSA analysts where there is a reasonable, articulable suspicion ("RAS"), based on specific facts."

If the government wants to take a closer look, any data gleaned must be associated with people or phone numbers already identified and approved by the secret Foreign Intelligence Surveillance Court. In 2012, the letter revealed, the court approved fewer than 300 "query terms" that would allow intelligence analysts to pursue a phone call further.

These protocols are overseen by the Justice Department and intelligence officials, and congressional intelligence committees are briefed regularly. "Thus, the program has been approved and is rigorously overseen by all three branches of the Government."

For these reasons, the program " is fully consistent with the Fourth Amendment," states the letter. "Most fundamentally, the program does not involve 'searches' of plaintiffs' persons or effects, because the collection of ... metadata from the business records of a third-party telephone service provider, without collecting the contents of plaintiffs' communications, implicates no 'legitimate expectation of privacy' that is protected by the Constitution."

The case may hinge upon how the court understands the kind of digital fingerprints most Americans leave behind with their phones everyday. In addition to the records of phone numbers and dates and times, GPS-enabled phones provide companies with tracking data and even records of a person's whereabouts and behavior.

"If the government were correct, then the NSA could collect now literally everything it could get its hands on in case a basis for searching the information one day materializes," says Alex Abdo, a staff attorney for the ACLU's National Security Project. "The government could, in short, create a digital catalog of the internet, our electronic communications, or even our physical movements and possessions in case it one day needs them.

"Needless to say, the Fourth Amendment does not permit such pervasive government monitoring," continues Mr. Abdo, who is also one of the lawyers arguing the case. "The government must demonstrate cause before it can track Americans' sensitive communications and movements."

In its letter, the Justice Department counters that "the program has contributed to the disruption of multiple potential terrorist attacks in the United States and abroad" while following strict constitutional and legal protocols.

And signaling a desire to be more forthcoming about the secret program, Director of National Intelligence James Clapper – the defendant named in the ACLU lawsuit – took the unusual step of announcing that the secret Foreign Intelligence Surveillance Court had renewed the sweeping telephone surveillance program "in light of the significant and continuing public interest." The program had been set to expire Friday.

For the ACLU, however, these steps don't address the fundamental issues at the heart of the case.

"The most troubling aspect of the government's response is its apparent belief that the Constitution does not apply to the NSA's collection of our call records but only to the government’s later searching of them," says Abdo. "In other words, the government believes in a collect-everything-now-and-ask-questions-later version of the Fourth Amendment, which is dramatically at odds with our nation’s history and the Supreme Court’s caselaw."