June 7, 2013, 6:49 p.m. ET

Data collected by the National Security Agency’s program that monitors Americans’ phone calls could be used to track millions of people’s locations through their mobile devices at any given time, according to people familiar with cellphone systems.

This is part of a new reality of surveillance: Advances in technology have vastly expanded the types of data included in records of calls and other communications. So even if the content of a call or email message isn’t seen, governments are able to glean details about where people are, what they are doing and with whom they are communicating.

Supporters of the NSA program in Congress said Thursday that the monitoring is legal precisely because it doesn’t involve the content of communications but relies on this call-record information known as “metadata.”

“Our courts have consistently recognized that there is no reasonable expectation of privacy in this type of metadata information,” Senate Intelligence panel leaders Dianne Feinstein (D., Calif.) and Saxby Chambliss (R., Ga.) said Thursday. President Barack Obama called the NSA’s activities “modest encroachments” on privacy on Friday.

In a separate program, dubbed Prism, the NSA collects data that includes content, but that program targets Internet communications involving at least one foreign person, the White House confirmed on Thursday after reports in two newspapers.

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