(CNN) -- The National Security Agency has built a network that can access as much as 75% of all U.S. Web traffic -- a larger amount than has been publicly announced -- according to a new report.
Citing current and former officials, The Wall Street Journal reported that the agency sometimes retains the content of e-mails between United States citizens and filters some domestic phone calls that use Internet connections.
A prepared statement to CNN from NSA spokeswoman Vanee Vines did not directly address the report's claims.
"NSA's signals intelligence mission is centered on defeating foreign adversaries who aim to harm the country," she said. "We defend the United States from such threats while fiercely working to protect the privacy rights of U.S. persons. It's not either-or. It's both."
The NSA had recently stated that it only "touches" 1.6% of the world's Internet data.
The agency has legal authority, working through wireless and other phone companies, to monitor calls coming into or going out of the country, as well as calls between other countries that are routed through the United States.
The Journal said its sources were current and former government and intelligence officials, as well as people working for companies that helped build, or provide data for, the monitoring system.
They said the way that system is set up makes it more likely that calls starting and ending in the United States are also intercepted and collected.
Former officials told the Journal there is a series of monitoring programs named Blarney, Fairview, Oakstar, Lithium and Stormbrew.
Vines told the Journal that if domestic calls are gathered while seeking out foreign ones, the agency uses "minimization procedures that are approved by the U.S. attorney general and designed to protect the privacy of United States persons."
The paper quotes another unnamed U.S. official saying the agency is "not wallowing willy-nilly. We want high-grade ore."
The NSA has been under heavy scrutiny since June, when former contractor Edward Snowden first leaked details about the agency's surveillance tactics.
Snowden claimed responsibility for leaking to The Guardian and other media outlets that the NSA had secretly collected and stored millions of phone records from accounts in the United States. The agency also collected information from U.S. companies on the Internet activity of overseas residents, he said.
Snowden is wanted on three felony counts, including violations of the U.S. Espionage Act. He fled first to Hong Kong and then to Russia, where he was granted temporary asylum despite pressure from the Obama administration to return him to the United States to face those charges.
Lawmakers have stepped up pressure in the wake of the leaks.
The House last month narrowly defeated a measure drafted by Michigan Rep. Justin Amash, a Republican, that would have restricted the NSA's surveillance program.
He told CNN on Sunday that he hopes the House will vote on the bill again.
"The system is not working," he said on CNN's "State of the Union."