NEW YORK—Signs grew that the economy could turn even weaker in 2009, as an index of December manufacturing activity sank to its lowest point in 28 years. Every corner of the sector was down, from bakeries to cigarette-makers to aluminum smelters.

The Institute for Supply Management, a trade group of purchasing executives, said Friday its manufacturing index fell to 32.4 in December, a greater-than-expected decline from November’s reading of 36.2. Wall Street economists surveyed by Thomson Reuters had expected the reading to fall to 35.5.

Components of the index hit hist...

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