Wholesale prices for cocaine have almost doubled in recent weeks, soaring from less than $20,000 a kilogram to more than $35,000, a result of drug dealers' price gouging, law enforcement officials said.

Investigators said that at the same time, the purity of cocaine sold on Washington area streets has dropped significantly.

"The importers seem to have decided it's time to give the illusion that cocaine is scarce," a law enforcement source said last week. "But really people are sitting on it to make it look scarce. It's the old game of supply and demand. Restrict the supply, and everyone makes more money.

"The importers are taking a page from what American oil companies are doing," the law enforcement source said.

Importers negotiating sales are saying that their supplies have been curtailed and that "the man is all over the place," referring to enforcement efforts, the source said.

Intelligence studies show that the amount of cocaine coming into the United States has decreased only slightly, sources said, but prices in this country are higher than at any time in the past five years.

In contrast to the $35,000 to $40,000 price for a kilogram lately, drug gang leader Rayful Edmond III usually paid $15,000 to $17,000 for a kilogram. A kilogram is 2.2 pounds.

Officials said the price increases are so recent that they aren't yet reflected in routine price and availability surveys regularly compiled by the Drug Enforcement Administration.

Capt. Collin Younger, head of the D.C. police narcotics branch, said his officers have recently paid $30,000 to $31,000 a kilogram in undercover drug investigations. DEA agents say purchases negotiated in recent weeks have been for $35,000 to $37,000 a kilogram.

"{Wholesale} prices for cocaine {powder} are definitely increasing, but for no reason that I can specifically put my finger on," Younger said last week. "The prices we see on the street are running pretty such the same . . . but the purity level has gone down."

Cocaine sold on the streets had been testing as much as 80 percent purity, but Younger said recent purchases have been around 60 percent and even as low as 50 percent.

Younger said that the low purity, especially in crack cocaine, may be the result of inexperienced people "cooking" the powdered cocaine into crack. If properly prepared, cooking removes additives put into powdered cocaine to cut or dilute it.

Younger said that laboratory analyses of crack bought in recent weeks have shown that "a lot of the additives . . . are still in the crack."

At the same time, "a lot of cocaine markets have dried up," Younger said, though he added dealers may have moved inside.

"Dealers aren't standing on the corners like they used to," Younger said. "There's definitely a change in the way it's being sold."

Yet the changes appear to have had a limited effect on users. The percentage of adult defendants in D.C. Superior Court who tested positive for cocaine use in July was 51 percent, up slightly from 48 percent in June, but about average for the past four months.

The July drug testing report of the D.C. Pretrial Services Agency shows that the percentage of all adult defendants testing positive for any drug was 54 percent, up slightly from the 52 percent reported in June. The percentage of defendants testing positive for PCP was 6 percent and 11 percent of defendants tested positive for heroin.

The increase in cocaine prices and decrease in purity come as the purity and availability of heroin in this area have increased markedly. According to federal and D.C. law enforcement agencies, the purity of heroin sold wholesale has risen to 80 percent to 98 percent, and street heroin is now routinely 30 percent to 40 percent pure.

The purity of both wholesale and street-sale heroin is now at least three times as much as it was two years ago, leading to a surge in the number of heroin overdose deaths, especially among those who use heroin in combination with cocaine, officials said.