One of the D.C. police department's top narcotics experts resigned suddenly this week amid allegations that he has lied under oath about his credentials.

Johnny St. Valentine Brown Jr. has offered his expertise in thousands of cases over the past 20 years, helping prosecutors win drug convictions. Among other things, he has testified that he has a doctorate in pharmacology from Howard University, a contention that suggests he has a special knowledge about the chemical makeup and workings of drugs. But Howard officials say they have no records that Brown even attended the school.

Barring the possibility of an unfortunate mix-up, Brown's days as a witness appear to be over. Police officials put Brown on administrative leave last week and were contemplating further action when he submitted his resignation Wednesday. The U.S. attorney's office has begun its own investigation.

If it turns out that Brown has committed perjury, defense lawyers said they could challenge convictions in cases in which prosecutors used him as a key witness.

"They were very proud of him and his credentials and the way he carried himself," said defense lawyer Bernard S. Grimm, a partner in the firm of Grimm, Petras & Wieser. "He was used and reused for the bigger cases, which now presents a real problem for them. It's going to have a domino effect."

Brown did not return telephone messages left at two recent addresses.

A charismatic speaker, Brown's testimony often made a big difference in court, Grimm and others said. He expressed opinions, for example, about whether defendants were carrying drugs for sale or personal use. The distinction can tilt juries to convict on drug distribution charges, which carry much longer prison terms than possession charges.

Brown, 56, who uses the nickname Jehru, was respected by area judges and opposing lawyers because he so clearly could describe the drug trade--from the slang used on the streets to the techniques used to package crack cocaine for resale. He formally retired as a detective in 1995 but was rehired by the department three days later on a contract basis. The arrangement enabled Brown to draw a $46,700 annual salary on top of his $65,000 police pension.

On the witness stand, Brown typically would begin his testimony by reciting details about his background, including his schooling. That, in turn, led judges to designate him as an expert witness.

No one is questioning Brown's law enforcement skills.

And no one apparently questioned his educational background, either, until lawyers in a pending civil lawsuit decided to verify whether he had a doctoral degree.

Brown was to be the expert witness for the District government in a lawsuit brought by the mother of Eric Butera, a police informant who was slain while trying to help homicide detectives solve the 1997 triple slayings at a Starbucks coffee shop in Northwest Washington. Government attorneys intended to use Brown to explain how police use informants and to describe the department's policies.

Butera's mother, Terry, filed a $115 million civil suit alleging that police failed to warn her son about the risks of working as an informant and then failed to protect him. The trial is scheduled to begin Oct. 5 in U.S. District Court. Brown was to testify that proper procedures were followed.

At a June 22 pretrial deposition, Peter Grenier, an attorney for the Butera family, asked Brown to state his highest level of education attained and when he reached it.

"Well," Brown replied, "my highest level of education would be a doctorate, 1972, in the field of pharmacology, Howard University."

"So you have a PhD in pharmacology?" Grenier asked.

"That's correct," Brown said, adding that he also had attained bachelor's and master's degrees in the same subject from the school.

After the deposition, Grenier contacted Howard.

In a June 30 letter, officials reported they couldn't find any records on Brown.

Grenier then accused the 28-year police veteran of perjury, filing court papers asking that he be disqualified as a government witness.

Attorneys for the District said Brown told them he would provide proof that he had the degrees. So far, Brown has come up with no documentation, law enforcement sources said.

Officials at Howard University said yesterday that they searched for Brown's records using his name and Social Security number and found nothing.

U.S. District Judge June L. Green has ruled Brown cannot testify as an expert in the Butera case.

At a hearing yesterday, Assistant Corporation Counsel Thomas L. Koger described the events as "a very serious problem and embarrassment."

The developments led to investigations by the D.C. police and the U.S. attorney's office. Channing D. Phillips, a spokesman for U.S. Attorney Wilma A. Lewis, confirmed the matter is "under internal review" and added, "It would be premature and inappropriate to comment any further at the present time."

Staff researcher Bobbye Pratt contributed to this report.