A study that set off a stampede for an experimental Alzheimer's disease drug four years ago was seriously flawed, and so far there's no evidence the drug is beneficial, the government said Thursday.

The U.S. Food and Drug Administration, accused of wrongly withholding the drug, has been reviewing evidence of its effectiveness since a researcher said in 1986 that it produced "encouraging initial results" in 17 Alzheimer's patients.Patients and their relatives immediately demanded access to the drug, called tacrine, or THA.

The FDA broke its customary silence on experimental drug reviews to bluntly criticize the researcher, Dr. William K. Summers, in a letter in the New England Journal of Medicine. The journal published Summers' original study.

The agency said Summers' report "led many to believe that a major breakthrough had been made in the treatment of Alzheimer's dementia."

"Unfortunately, an FDA investigation . . . has revealed deficiences of sufficient importance to raise major questions about the validity of the conclusions," the letter said.

The agency said, however, that it "did not uncover clear evidence" that the researcher "had purposely misrepresented his work."

The journal published a response from the doctor:

"The letter from the FDA is more in the nature of a personal attack on our group than of a scientific critique designed to advance understanding of the present-day importance of our work," Summers said.

Nancy Newman, a spokeswoman for Summers, said he would not comment further.

Don McLearn, a spokesman for the FDA, declined to comment on the substance of the dispute. But he acknowledged that it was unusual to comment on a drug still under review. He said the decision to do so was made in response to a barrage of criticism accusing the agency of wrongly withholding the drug.