The Economist has a very depressing article that describes the range and depth of the LIBOR disaster: The rotten heart of finance. If you are following LIBOR scandal, you will really want to read this article. Be forewarned it makes for sad reading.
There are two different ways LIBOR rates have been manipulated.
First is a longer running and less severe manipulation. For number of years, at least since 2008, perhaps as long ago as 2005, and perhaps even earlier, the article says traders inside the banks have been moving the rate a little bit. Second is a larger amount of movement for a shorter period of time. This took place during the financial crisis of 2008 in order to understate borrowing costs.