WASHINGTON - Residential heating costs, already expected to be higher this winter than last, could take another jump after natural gas prices soared by nearly 50 percent within the past three weeks.

Analysts weren't sure what caused the sudden and steep increase in wholesale natural gas prices, but said that an early winter storm, speculation and concern about tight, although adequate, supplies all played a part.

The spot price of gas at the Henry Hub trading center in Louisiana closed at $6.59 a thousand cubic feet on Tuesday, which was somewhat lower than the day before but substantially higher than the $4.45 price just before Thanksgiving.

"The bottom line is you've got a jittery market," said Paul Wilkinson, an economist for the American Gas Association, which represents gas utility retailers. While utilities buy only a small part of their gas on the spot market, the higher fuel prices likely will be passed on to their customers, he acknowledged.

"There's no question. ... The prices today will get passed on. We're just in a sense a middleman," said Wilkinson.

Industrial gas users also are worried.

"At the moment, the price of natural gas is serving as the single largest brake on the U.S. economy," says Greg Lebedev, president of the American Chemistry Council, whose members use large amounts of gas to make pesticides and other products.

The surge in gas prices prompted calls in Congress for an investigation into possible price gouging and market manipulation.

Sen. Orrin Hatch, R-Utah, who heads the Senate Judiciary Committee, said he couldn't understand why prices would soar as they have when gas inventories are above average for this time of year.

"We must determine once and for all if these price surges are the result of market forces or if there continues to be price manipulation," Hatch said, promising hearings into the price spikes.

Energy Secretary Spencer Abraham told reporters Tuesday that weather has been a factor in the price surge, but "it may not be the only factor." Still, he cautioned against "premature finger pointing" and warned of continued price volatility in natural gas markets if production is not increased.

While current stocks - nearly 3 trillion cubic feet, or just over the five-year average for this time of year - appear to be adequate they are not at a level of comfort, Abraham said.

Analysts said a combination of events have spooked the market.

Among them were an onslaught of severe weather in the Northeast and Midwest, the memory of similar conditions a year ago when an unexpected cold spell caused a sudden drop in supplies and soaring prices, and market speculation.

Some traders, who thought prices would moderate this winter, recently shifted gears amid concerns that prices would remain high and moved to cover any potential losses, driving prices up further, some analysts said.

While both the Federal Energy Regulatory Commission and the Commodity Futures Trading Commission are keeping a close watch on natural gas markets, neither agency has indicated finding anything illegal.

"We watch the markets on a regular basis. We're certainly aware of the situation," FERC spokesman Brian Lee said. CFTC spokesman David Gary said such monitoring was not unusual when there were dramatic movements up or down in a market.

But some analysts argued there was no reason for the steep price jumps.

"Traders are hyping the markets. There are producers that are hyping the potential shortages. ... I don't want to call it a conspiracy, but all of these players are operating in concert," said Fadel Gheit, senior energy analyst at Oppenheimer & Co.

Earlier this year, the Energy Information Administration estimated that average heating costs for families would be about 6 percent higher this winter, or about $850 for the heating season.

David Costello, an EIA economist, said that assumed $5 natural gas and a normal winter. More recently the agency increased the estimate to 10 percent higher than last winter. But that estimate did not take into account the latest price surge.

The problem, said Costello, is that the early cold spell has traders worried that the cushion provided by the current inventories might disappear rapidly with another frigid winter surge.

"If we get through this month and (gas) storage is still normal the guess is that prices will correct downward," said Costello.

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On the Net:

Energy Information Administration: www.eia.doe.gov

American Gas Association: www.aga.org