The original author of the Patriot Act says the government's "excessive" use of it to collect the phone records of millions of Verizon customers is an abuse of power.
"While I believe the Patriot Act appropriately balanced national security concerns and civil rights, I have always worried about potential abuses," Rep. Jim Sensenbrenner, a Wisconsin Republican, said in a statement.
"Seizing phone records of millions of innocent people is excessive and un-American."
And in a scathing letter to Attorney General Eric Holder, Sensenbrenner added: "These reports are deeply concerning and raise questions about whether our constitutional rights are secure. …
"How could the phone records of so many innocent Americans be relevant to an authorized investigation as required by the Act?"
Sensenbrenner's remarks came a day after London's Guardian newspaper revealed the National Security Agency had been collecting the records under a court order.
"This is a big deal, a really big deal," Sensenbrenner told Fox News, noting that the broad seizure of the call records was "never the intent" of the law.
He added that he may try to amend the Patriot Act before it expires in 2015 to prevent further alleged abuses.
Not all of Sensenbrenner's GOP colleagues agree with him.
Rep. Mike Rogers, a Michigan Republican and chairman of the House intelligence committee, insisted the program has been already used to effectively stop a terrorist attack on U.S. soil.
Sen. Saxby Chambliss, a Georgia Republican, said the mining of phone records to assist in terrorism probes has been going on for seven years.
"Everyone should just calm down," Senate Democratic Leader Harry Reid said.
On Friday, the Electronic Privacy Information Center, a nonprofit privacy advocate, asked Congress to launch a series of oversight hearings on whether the collection was legal.
The center said the domestic surveillance was "unrelated" to the government's scrutiny of foreign intelligence related to terrorism.
An order signed by a federal judge in April requires Verizon to deliver "on a daily basis" to the National Security Agency all information about all domestic and overseas calls, "including local telephone calls."
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The judge also ordered Verizon not to go public with its handing over the records.
Friday afternoon, President Barack Obama defended the surveillance, saying that Congress has repeatedly authorized it.