BOGOTA, Colombia (AP) – The street price of cocaine fell in America last year as purity rose, the White House drug czar said in a private letter to a senator, indicating increasing supply and seemingly contradicting American claims that $4 billion in aid to Colombia is stemming the flow.

The drug czar, John Walters, wrote that retail cocaine prices fell by 11 percent from February 2005 to October 2006, to about $135 per gram of pure cocaine – hovering near the same levels since the early 1990s. In 1981, when the U.S. government began collecting data, a gram of pure cocaine fetched $600.

Analysis of data collected by the American Drug Enforcement Administration showed that after a drop in 2005, levels of purity “have trended somewhat toward former levels,” Mr. Walters said.

Price and purity estimates are a key barometer of cocaine availability. Dropping prices are an indication of robust supply or weakening demand, as is rising purity.

Mr. Walters made the disclosure in a January letter to Senator Grassley of Iowa, the Republican co-chair of the Senate Caucus on International Narcotics Control. The Washington Office on Latin America, a lobby group, obtained the letter and made it available to The Associated Press.

Rafael Lemaitre, a spokesman for the White House Office of National Drug Control Policy, told the AP that Mr. Walters would not comment on the letter, but Mr. Lemaitre described it as “an accurate reflection of our agency’s thoughts on the issue.”

Next Wednesday, President Uribe is set to meet with President Bush at the White House to discuss American support for Plan Colombia, the anti-narcotics and counterinsurgent program that has cost American taxpayers more than $4 billion since 2000.

American officials have insisted that Plan Colombia is reducing the quality and availability of cocaine in America, which gets 90 percent of its cocaine from Colombia.

But Mr. Grassley, in an e-mailed statement to The Associated Press, said the letter is “all the proof that anybody needs” that the White House drug office “has gotten quite good at spinning the numbers, but cooking the books doesn’t help our efforts to curb cocaine and heroin production and consumption.”

The numbers cited by Mr. Walters contradict upbeat appraisals made by American officials as recently in March – two months after Mr. Walters’ letter.

Several household and school-based surveys show that America’s cocaine consumption has barely budged since 2000, even as drug use in Europe, which also affects supply, has soared.

Representative Jim McGovern, Democrat of Massachusetts, said despite the existence of the new estimates, senior American Embassy officials provided him with older, more encouraging data during a March visit to Bogota.

“We’ve given this program a chance to work and clearly this is not producing the results we were promised,” he said. “Cocaine is priced as low and purity is as high as it was before Plan Colombia began six years and $5 billion ago.”

And despite a record fumigation of almost 550 square miles in 2005, 26 percent more land was dedicated to production of the plant used to make cocaine. The 2006 estimates are to be released in May.

In November 2005, Mr. Walters said cocaine prices had risen by 19 percent and purity had dropped by about the same. He touted the development as a sign that America had turned the corner in anti-drug efforts. Drug policy experts rejected his assertions at the time, and Grassley called for his dismissal.

Mr. Walters’ latest letter to Mr. Grassley came in response to a request from the Iowa senator.

“When the data show a brief rise in cocaine prices, the drug czar holds a high-profile press conference,” said Adam Isacson, an analyst at the Washington-based Center for International Policy. “But when the trend goes back down again, the drug czar sends it in a letter to one senator. Why is that?”