The Fed has been threatening to raise rates for more than half a decade.
In 2009, then Fed Chairman Ben Bernanke laid out the road map as to WHEN quantitative easing (QE) would end, interest rates would be raised and the Fed would unwind its balance sheet:
While doing a few rounds of multi trillion dollar quantitative easing programs, former Fed Chairman Ben Bernanke determined that, despite constantly referring to a "recovery", the economy was too weak to raise rates during his tenure.
Mr. Bernanke claimed that the economy was weak in 2012. In 2013, Bernanke said the economy was weak and added that it would "tank" if rates were raised.
Current Fed President Janet Yellen takes a different tact - she has been claiming all throughout 2014 that the economy IS better and it's just a matter of "considerable time" until the Fed raises rates. The Fed can not raise rates because it would crash the asset bubbles in the stock and real estate markets and raise the cost of U.S. deficit spending. The Fed can't raise rates because other central banks have low or negative interest rates and are also monetizing or considering monetizing their sovereign debts and other stimulus measures.