Millions felt the pinch last week as another wave of biting, bitter cold strained already low propane supplies in the Midwest, causing prices to surge three-fold over two weeks to record highs and forcing suppliers to ration deliveries.

While the shortage was likely caused by a confluence of events, including extraordinary cold, a temporary pipeline shut-down, and low stocks, some industry observers and at least one politician blamed the spike in part on soaring exports of liquefied petroleum gases (LPGs) such as propane.

Unlike crude oil and natural gas, both of which are now at the center of a national debate over restrictive export rules, LPG and other fuels can be freely shipped abroad.

Ron Wyden, Senate Energy Committee Chairman and an Oregon Democrat who has opposed unlimited exports that could raise consumer prices, believes the propane squeeze is a "concrete example" of the risks that consumers may face, he said via a spokesman. It shows why "the United States needs to take a cautious approach to energy exports".

Iowa Senator Chuck Grassley called on the Federal Trade Commission to review the cause of the price spike to ensure markets weren't being manipulated.

By Friday evening, wholesale prices had begun to subside, offering hope that the shortage may be easing as traders ship cargoes of compressed LPG to the East Coast and truckers divert more supplies from the Gulf Coast refining hub.

Still, the scare may fuel more resistance to easing export constraints of any type of energy, and shows how the unexpected eruption of U.S. shale oil and gas is also creating temporary pockets of scarcity, as energy companies adjust their supply chains and transport networks to a new era of abundance.

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Propane production has grown across the country from shale wells that typically produce LPGs alongside oil and gas, rising 27 percent over the past five years to 1.4 million barrels per day (bpd), Energy Information Administration data shows.

As output rose, prices slumped to well below global rates, because producers lacked the specialized, pressurized facilities to ship propane abroad. By mid-2012, benchmark prices in Conway, Kansas, fell to 50 cents a gallon.

But with a profitable price gap, companies including Enterprise Product Partners (EPD.N) and Phillips 66 (PSX.N) invested millions in export facilities, and shipments have swelled. After rising to 170,000 bpd on average in 2012, exports reached 300,000 bpd last May and topped 400,000 bpd for the first time in October, EIA data show.

Politicians and energy analysts explain the shortage as the culmination of several unusual factors - a record-breaking freeze at the start of January came just as propane inventories were abnormally low due to a bumper corn harvest last fall, which used up propane for drying out the crop.

A pipeline outage during most of December exacerbated the situation.

Nonetheless, opponents of energy exports believe there is more to the coincidence of an export boom with a price spike.

"I know some want to characterize the situation as the perfect storm," said Brandon Scholz, managing director of the Wisconsin Propane Gas Association. "But we believe that less and less. ... The high level of exports came at a time when they contributed to a significant fall in propane inventories."