The PRISM special source operation system legally immunized private companies that cooperate voluntarily with U.S. intelligence collection. According to "The Register", the FISA Amendments Act of 2008 "specifically authorizes intelligence agencies to monitor the phone, email, and other communications of U.S. citizens for up to a week without obtaining a warrant" when one of the parties is outside the U.S. PRISM was first publicly revealed on 6 June 2013, after classified documents about the program were leaked to "The Washington Post" and "The Guardian" by American Edward Snowden.