Accused Colombian cocaine wholesaler Marcos Cadavid was convicted of conspiracy yesterday by a U.S. District Court jury here, giving the Justice Department proof of its power to extradite and prosecute suspected South American drug traffickers.

Of four defendants extradited to the United States in January under a new antitrafficking treaty with Colombia, Cadavid was the first to go on trial.

"We have successfully completed the international investigatory cycle by returning to this country and convicting a significant drug trafficker," U.S. Attorney Joseph E. diGenova said of Cadavid yesterday.

"The government and people of Colombia can now be assured the American process of justice works fairly and safely." He called the verdict "an eloquent postscript" to the murder of a U.S. Drug Enforcement Administration agent in Mexico.

Cadavid's six-day trial at the federal courthouse here was conducted under unusually heavy security following what law enforcement sources said were unspecified death threats from revenge-minded Latin drug dealers. U.S. District Judge Thomas F. Hogan ordered the jury sequestered during the trial for security reasons.

Cadavid, 43, was convicted of a single count of conspiracy to import and distribute cocaine between 1976 and 1983. He faces a maximum penalty of 15 years in prison and a $25,000 fine. No sentencing date was set by Hogan.

According to prosecution testimony, Cadavid and a second Colombian, Armando (The Hammer) Marulanda, formed a Miami connection, supplying scores of kilograms of cocaine to a D.C.-based drug ring during meetings at a Miami apartment. Marulanda is a fugitive.

Four former members of the ring, all of whom have pleaded guilty, testified they paid Cadavid and Marulanda a total of about $20 million for the drugs over the seven-year period.

There was no evidence that Cadavid, who declined to take the witness stand, traveled to the District in connection with the drug scheme.

"Cadavid never came here," Assistant U.S. Attorney Roger M. Adelman told the jury, "but his cocaine did, every month, every week.

"That dirty white powder he sells comes here, and that speaks louder than he can."

Cadavid's lawyer, Melvyn Kessler of Miami, attacked the credibility of the government's witnesses and what he called "sweetheart deals" they made in bargaining with prosecutors to testify in exchange for the hope of lighter sentences.

"Reduced exposure [to prison] -- that was the purpose of telling the dream that they told," Kessler said. "The government doesn't always have ministers, priests and rabbis as witnesses."

Seven members of the Washington-based ring, including its leader, Lawrence G. Strickland Jr. of Bethesda, have pleaded guilty. Four others, among them former lobbyist Fred B. Black Jr. of Washington, were convicted in federal court here 10 days ago.

Black and William G. Hessler, a former vice president of Riggs National Bank, are scheduled to begin trial here on March 25 on multiple banking, fraud and racketeering charges in connection with alleged laundering of the ring's drug profits.