"As you know, this is just metadata," Feinstein said at a news conference on Thursday. "There is no content involved."
Asked if it's time to "revist" some of the legislation that allows such government intrusion, Feinstein said "people are trying to get to us."
"This is the reason why we keep TSA doing what it's doing. This is the reason why the FBI now has 10,000 people doing intelligence on counterterrorism. This is the reason for the National Counterterrorism Center that's been set up in the time we've been active. And it's to ferret this out before it happens. It's called protecting America."
According to Feinstein, the information "goes into a database...but cannot be accessed without what's called, and I quote, 'reasonable, articulable suspicion' that the records are relevant and related to terrorist activity."
She said the surveillance is "lawful" and "has been briefed to Congress."
The committee's ranking Republican, Saxby Chambliss (R-Ga.) told reporters, "This is nothing particularly new. This has been going on for seven years under the auspices of the FISA authority, and every member of the United States Senate has been advised of this, and to my knowledge, we have not had any citizen who has registered a complaint relative to the gathering of this information.
"And it is simply what we call metadata that is never utilized by any governmental agency unless they go back to the FISA court and that there is real cause as to why something within the metadata should be looked at. So that's, I think, been very clear all along through the years of this program. It has proved meritorious because we have gathered significant information on bad guys, but only on bad guys over the years."
Feinstein said she understands Americans' growing privacy concerns, but repeated that the phone surveillance is done only with approval of a federal court.
Feinstein described the metadata as "a telephone book of the numbers."
She said if the FBI obtains other information pointing to a terrorist act, "they can access those records. The records are there to access."
In response to a question, Feinstein said she does not know how long the metadata can be kept.
Chambliss said the information the National Security Agency is seeking "is on the other end of the call. It's, are they in contact with folks who aren't -- is somebody in contact with somebody that we know to be a known terrorist? And that's why it's metadata only and it's what we call minimized.
"All of these numbers are basically ferreted out by a computer, but if there's a number that matches a terrorist number that has been dialed by a U.S. number or dialed from a terrorist to a U.S. number, then that may be flagged. And they may or may not seek a court order to go further on that particular instance. But that's the only time that this information is ever used in any kind of substantive way," Chambliss said.