The United States government says it searched for detailed information on fewer than 300 phone numbers last year.
A government paper said they were among millions of phone and email records collected by the National Security Agency.
The paper said that such searches led to two men who were plotting to attack New York's subway system in 2009.
The newly declassified paper has just been circulated within the government by US intelligence agencies. It was made public by the Senate Intelligence Committee on Sunday.
The BBC reports the document appears to be an attempt to counter accusations that the government went too far in investigating potential terror plots.
The administration insists that even though the NSA collects massive amounts of data from US telephone and internet companies, such data collection is legal, subject to tight controls and does not intrude on the privacy of ordinary Americans. It says the data is destroyed every five years.
The monitoring programmes revealed by a series of stories in The Guardian and The Washington Post.
The Guardian newspaper says the British government monitored phone calls and internet use of foreign politicians at the G20 meeting in London in 2009.
The report is based on documents leaked by the US National Security Agency contractor Edward Snowden.
The newspaper said some delegates were tricked into using fake internet cafes set up so intelligence officers could read their emails.
The Guardian also claimed that American spies based in Britain intercepted the communications of then-President Dimitri Medvedev of Russia during his visit for the summit.