WASHINGTON—The National Security Agency—which possesses only limited legal authority to spy on U.S. citizens—has built a surveillance network that covers more Americans' Internet communications than officials have publicly disclosed, current and former officials say.

The system has the capacity to reach roughly 75% of all U.S. Internet traffic in the hunt for foreign intelligence, including a wide array of communications by foreigners and Americans. In some cases, it retains the written content of emails sent between citizens within the U.S. and also filters domestic phone calls made with Internet technology, these people say.

The NSA's filtering, carried out with telecom companies, is designed to look for communications that either originate or end abroad, or are entirely foreign but happen to be passing through the U.S. But officials say the system's broad reach makes it more likely that purely domestic communications will be incidentally intercepted and collected in the hunt for foreign ones.

The programs, code-named Blarney, Fairview, Oakstar, Lithium and Stormbrew, among others, filter and gather information at major telecommunications companies. Blarney, for instance, was established with AT&T Inc., former officials say. AT&T declined to comment.

This filtering takes place at more than a dozen locations at major Internet junctions in the U.S., officials say. Previously, any NSA filtering of this kind was largely believed to be happening near points where undersea or other foreign cables enter the country.