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WASHINGTON — A plunge in the cost of gas drove down a measure of U.S. consumer prices last month by the most since December 2008. Excluding the drop in fuel costs, prices across the country were largely unchanged.

In the Seattle area, prices rose 0.4 percent between February and April 2013, the slowest gain for those months in three years. Local prices are up 1.2 percent over the past 12 months.

Seattle-area food has gone up 2.9 percent since April 2012.

Local gasoline pries fell by 10.7 percent in that same time, although between February and April this year, they have gained 0.5 percent. And in recent weeks, gas has been climbing ever higher — including a 22-cent gain to $3.91 in Washington last week alone.

One of the few categories to drop since February was alcoholic beverages, which fell 0.4 percent in the Seattle area.

Nationally, the consumer price index fell 0.4 percent in April from March, the Labor Department said Thursday. The main reason the index fell was that gas prices plunged 8.1 percent.

For the 12 months that ended in April, overall prices rose 1.1 percent — the smallest year-over-year increase in 2½ years.

Excluding volatile energy and food costs, “core” prices ticked up 0.1 percent last month. Core prices have risen only 1.7 percent in the past 12 months. That’s just below the Federal Reserve’s 2 percent inflation target.

Scant inflation is allowing the Fed to continue its extraordinary efforts to stimulate the economy. Worries about lower inflation or even deflation might push the Fed to step up its low interest-rate policies to stimulate more borrowing and spending and push prices higher.

Deflation is a destabilizing cycle in which prices and wages fall steadily. It can slow economic growth.

Unusually low inflation means consumers can stretch their paychecks and buy more goods and services. But if it were to fall further, it could stoke fears of deflation.

“Subdued demand means that core inflation is likely to edge lower, as retailers will be forced to pass previous falls in raw material costs onto customers,” Paul Dales, an economist at Capital Economics, said in a note to clients. “The Fed may soon put more emphasis on fading inflation trends.”

A little inflation can be good for the economy, because it encourages businesses and consumers to spend before prices rise further.

Aside from sharp swings in gas prices, consumer and wholesale inflation has been mild this year. The combination of modest economic growth and high unemployment has kept wages from rising quickly. That’s made it harder for retailers and other firms to raise prices.

The average national price for a gallon has fallen since reaching a peak this year of $3.79 on Feb. 28. The average price was $3.60 a gallon on Thursday, according to AAA.

Seattle Times business reporter Melissa Allison contributed to this report.