WASHINGTON -- The head of the Senate Intelligence Committee said Thursday that the panel would investigate reports that the National Security Agency improperly tapped into the domestic communications of American citizens. 

The Justice Department confirmed Wednesday that it had reined in the NSA's wiretapping activities in the United States after learning that the agency had improperly accessed American phone calls and e-mails while eavesdropping on foreign communications. 

Senate Intelligence Committee Chairman Dianne Feinstein, a Democrat, said the committee will hold a hearing within a month to look at the NSA's surveillance activities. "We will make sure we get the facts," she said. 

The Justice Department discovered the problems during a routine review of NSA wiretapping. The government's action was first divulged Wednesday by The New York Times. 

Department officials said the problems have been corrected, but they declined to say what measures were taken. Justice officials would not detail how the law governing NSA wiretapping was violated or for how long and how many Americans' communications were compromised. 

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Critics of the secret program -- the extent of which has never been revealed -- contend the government has illegally wiretapped and used data-mining techniques to sweep up vast amounts of phone and e-mail communications. 

The government's secret use of domestic eavesdropping has been a contentious issue since 2005, when it was revealed that for years following the Sept. 11, 2001, terrorist attacks, President George W. Bush gave wide authorization for the NSA to intercept phone conversations and e-mails involving U.S. citizens without a warrant. 

The Foreign Intelligence Surveillance Act, adopted 30 years ago, requires the government to obtain permission from a secret FISA court before wiretapping American phone lines, and also covers computer lines. The law was intended to prevent the political abuse of government surveillance powers. 

Congress changed the law last year, both loosening some provisions and tightening other in an effort to strike a balance between protecting national security and guarding civil liberties.