According to a declassified Justice Department report, the Federal Bureau of Investigation (FBI) has been keeping the NSA's email surveillance program under close scrutiny but the details about how the bureau administered remains unclear.
According to the Verge, FBI began overseeing the NSA's surveillance program in 2008 to keep a watch that the email accounts it targeted belonged to non-U.S. citizens living abroad.
The bureau in 2009 started collecting communications for its own purposes and began adding new email accounts and phone numbers through NSA's upstream data collection program.
The FBI said in its study that it is doing a good job limiting warrantless searches to non-American email accounts.
U.S. whistleblower Edward Snowden also revealed the inner workings of the NSA's surveillance programs.