The head of the National Security Agency on Thursday told Senate lawmakers that preventing his agency from collecting Americans’ information in bulk would make it harder to do its job.

Under questioning before the Senate Intelligence Committee, Adm. Michael Rogers agreed that ending bulk collection would “significantly reduce [his] operational capabilities.”

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“Right now, bulk collection gives us the ability ... to generate insights as to what’s going on,” Rogers told the committee.

The NSA head also referenced a January report from the National Academy of Sciences that concluded there is “no software technique that will fully substitute for bulk collection” because of the ability to search through the storehouse of old information.

“That independent, impartial, scientifically-founded body came back and said no, under the current structure there is no real replacement,” Rogers said.

Rogers was questioned on Thursday by Sen. Ron Wyden Ronald (Ron) Lee WydenGOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate GOP senator blocks Schumer resolution aimed at Biden probe as tensions run high MORE (D-Ore.), a member of the Intelligence Committee who has become its most vocal privacy hawk.

In response to the NSA head’s comments, Wyden pointed to a 2013 White House review group, which found that one controversial NSA bulk collection program “was not essential to preventing attacks” and that the information obtained by the NSA “could readily have been obtained in a timely manner using” other means.

The debate follows on a congressional clash earlier this year over the NSA’s bulk collection of records about the phone calls of millions of Americans. The records contained information about whom people called and when but not what they talked about.

After a brief lapsing of some portions of the Patriot Act, Congress eventually reined in the NSA by forcing it to go through the courts to search private phone companies’ records for a narrower set of records.

Many privacy advocates treated the new law, called the USA Freedom Act, as a significant victory, through national security hawks worried that it would make it harder for the NSA to track terrorists.

Under the new system — which has not gone into effect yet — the amount of time it takes to obtain those records “is probably going to be longer I suspect,” Rogers said.

Though the phone records database has been the NSA’s most prominent bulk collection program, it is not the only one. The agency’s collection of vast amounts of Internet data has alarmed many privacy advocates and is the target of a current lawsuit from Wikipedia and the American Civil Liberties Union.