U.S. stock index futures indicated a weaker open on Wall Street on Tuesday before a key economic report forecast to show a dip in manufacturing activity last month.

Investors returned from the Labor Day holiday to the first trading day of September, marking the unofficial end of summer and the start of an historically negative month for stocks.

Shares of Research in Motion Ltd. fell before the opening bell on Tuesday after Bear Stearns cut its rating on the maker of the Blackberry device, according to theflyonthewall.com.

Research in Motion shares were down 2.2 percent to $83.50.

Stocks rebounded on Friday, bolstered by optimism about a White House-proposed mortgage assistance plan and Federal Reserve Chairman Ben Bernanke's assurance the central bank will take the necessary steps to shelter the economy from turmoil in financial markets.

The Institute for Supply Management is set to report the August reading of its manufacturing index at 10 a.m. EDT. Economists forecast a dip in the indicator to 53 last month from 53.8 in July.

U.S. construction spending and auto sales data are also scheduled for Tuesday.

We're looking at a lower opening. We're really going to be scrutinizing the economic numbers, said Peter Cardillo, chief market economist at Avalon Partners in New York. If we saw real weakness in the ISM or car sales, that would stir up the debate over how big a Fed interest rate cut we get.

S&P 500 futures were down 3.4 points, below fair value, a mathematical formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract.

Dow Jones industrial average futures fell 24 points, and Nasdaq 100 futures fell 3.75 points.

September's stock trading, going back to 1980, has seen more declines than rises, with the market dropping 15 times versus 12 gains.

The average September return for the S&P 500 index has been a drop of 0.95 percent. Figures are price returns on the index and do not include dividend returns.

Shares of Eli Lilly and Co could benefit after researchers said its experimental schizophrenia treatment appeared to work safely in patients without some of the side effects of older drugs.

Dow component Merck & Co will also likely be in the spotlight after researchers said on Sunday that the pharmaceutical company's combination pill to raise HDL cholesterol without a common side effect, met its main goals in a late-stage test.