Stock index futures turned lower on Friday on news the U.S. government is taking a large common equity stake in embattled lender Citigroup .

There was also caution ahead of key economic reports, including one that may show that the U.S. economy contracted more than initially thought in the fourth quarter.

S&P 500 futures fell 8.30 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures lost 52 points, and Nasdaq 100 futures were down 5.75 points.

(Reporting by Ellis Mnyandu; Editing by James Dalgleish)