The NSA program disclosure comes at a time of unprecedented cyberattacks on U.S. financial institutions, businesses and government agencies.

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WASHINGTON — Without public notice or debate, the Obama administration expanded the National Security Agency’s (NSA) warrantless surveillance of Americans’ international Internet traffic to search for evidence of malicious computer hacking, according to classified NSA documents.

In mid-2012, Justice Department lawyers wrote two secret memos permitting the spy agency to begin hunting on Internet cables, without a warrant and on U.S. soil, for data linked to computer intrusions originating abroad, including traffic that flows to suspicious Internet addresses or contains malware, the documents show.

The Justice Department allowed the agency to monitor only addresses and “cybersignatures” — patterns associated with computer intrusions — that it could tie to foreign governments.

But the documents also note that the NSA sought permission to target hackers even when it could not establish links to foreign powers.

The disclosures, based on documents provided by Edward Snowden, the former NSA contractor, and shared with The New York Times and ProPublica, come at a time of unprecedented cyberattacks on U.S. financial institutions, businesses and government agencies — and of greater scrutiny of secret legal justifications for broader government surveillance.

While the Senate passed legislation this week limiting some of the NSA’s authority, it involved provisions in the USA Patriot Act and did not apply to the warrantless-wiretapping program.

Government officials defended the NSA’s monitoring of suspected hackers as necessary to shield Americans from the increasingly aggressive activities of foreign governments. But critics say it raises difficult trade-offs that should be subject to public debate.

The NSA’s activities run “smack into law-enforcement land,” said Jonathan Mayer, a cybersecurity scholar at Stanford Law School who has researched privacy issues and who reviewed several of the documents. “That’s a major policy decision about how to structure cybersecurity in the U.S. and not a conversation that has been had in public.”

It is not clear what standards the agency is using to select targets. It can be hard to know who is behind a particular intrusion — a foreign government or a criminal gang — and the NSA is supposed to focus on foreign intelligence, not law enforcement.

The government can also gather significant volumes of Americans’ information — anything from private emails to trade secrets and business dealings — through Internet surveillance because monitoring the data flowing to a hacker involves copying that information as the hacker steals it.

One internal NSA document notes that agency surveillance activities through “hacker signatures pull in a lot.”

Brian Hale, spokesman for the Office of the Director of National Intelligence, said: “It should come as no surprise that the U.S. government gathers intelligence on foreign powers that attempt to penetrate U.S. networks and steal the private information of U.S. citizens and companies.”

He added that “targeting overseas individuals engaging in hostile cyberactivities on behalf of a foreign power is a lawful foreign intelligence purpose.”

The effort is the latest known expansion of the NSA’s warrantless surveillance program, which allows the government to intercept Americans’ cross-border communications if the target is a foreigner abroad.