U.S. National Security Agency Director General Keith Alexander says NSA surveillance programs have helped prevent more than 50 terrorist attacks since 9/11.
He said two recently disclosed surveillance programs that gather U.S. phone records and track the use of U.S.-based Internet servers by foreigners with possible links to terrorism "are critical to protecting our country."
General Alexander gave a congressional committee some information on two cases he said the programs thwarted. One a plot from Yemen to bomb the New York Stock Exchange and another that involved providing funding for a Somalia-based terrorist group.
He said he would give the committee documents about the rest of the cases Wednesday in a classified setting.
Deputy Attorney General James Cole also testified that there are several checks for abuse in the surveillance programs and they have oversight by all three branches of government.
The committee chairman, Republican Mike Rogers, and top Democratic member C.A. Dutch Ruppersberger agreed the programs are legal as well as critical. They criticized the recent leaks about the programs, with Rogers calling it "criminal behavior".
On Monday, U.S. President Barack Obama repeated earlier assertions the NSA programs were a legitimate counterterror tool and completely noninvasive to people with no terror ties.
In a PBS television interview, he insisted the NSA phone and Internet monitoring efforts operate within the law.
National intelligence legal chief Robert S. Litt said under the programs any information collected on a conversation because of an error must be purged from the system.