He said that in 53 of the 54 cases, the agency also had used its authority to eavesdrop on internet traffic of foreign intelligence targets. The actions allowed the US to collect data which "played a critical role."
Alexander said that almost half of NSA's counter-terrorism reporting came from internet monitoring.
The Guardian report said that NSA collection of internet metadata initially began under a controversial warrantless wiretapping program authorized by the administration of President George W Bush, but was later authorized by the secretive Foreign Intelligence Surveillance Court.
The Guardian quoted Shawn Turner, chief spokesman for the US Director of National Intelligence, saying that the collection program, which continued after Barack Obama became president, was terminated in 2011 "for operational and resource reasons and has not been re-started."
Now shut down
In his comments, which apparently responded to the latest Guardian story, Alexander acknowledged that NSA's email metadata collection program had been "analogous" to its telephone call data collection. He said the program had ceased because "it didn't have the operation impact that we needed."
"Because it wasn't meeting what we needed and we thought we could better protect civil liberties and privacy by doing away with it, Alexander said. He added: "And all that data was purged at that time."
However, the Guardian said that it had seen other secret NSA papers suggesting that some online data collection continued today.
Previously, citing documents provided by former NSA contractor Edward Snowden, the Guardian published a secret court order authorizing NSA to collect masses of similar "metadata" charting traffic between phone numbers. The order, covering calls both within the United States and between the United States and foreign countries, had been re-issued as recently as April 2013.
The Guardian's latest story said that when NSA began collecting Internet metadata in 2001, the agency was limited to only cases where "at least one communicant" was located outside the United States "or for which no communicant was known to be a citizen of the United States."
However, the Guardian printed what it said was a 2007 secret US Justice Department memo. It said the memo indicated that NSA later got authority to "analyse communications metadata associated with United States persons and persons believed to be in the United States."
A top secret draft report on the email program prepared by NSA's inspector general, and posted on the Guardian's website, makes clear the key role played by private companies, who partner with NSA. In the report's words, they help the US government "obtain access to information that would not otherwise be available."
The report describes how, in early October 2001, NSA officials approached three major US-based communication service providers - identified in the report as COMPANY A, COMPANY B, and COMPANY C - to seek cooperation with the new surveillance program. "Each company agreed to cooperate," the report says.
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