Influential D.C. contractor John B. Clyburn was acquitted yesterday on federal bribery and conspiracy charges, bringing to a close a long and controversial investigation and trial in which prosecutors alleged he manipulated the District's contracting process.

The verdict came four days after Clyburn's codefendant, David E. Rivers, the former head of the city's largest agency, was acquitted of the same charges. The jury, which deliberated about 20 hours before acquitting Rivers, went back to work for 19 more hours before finding Clyburn not guilty of manipulating the contract process to win business for himself and his friends.

The acquittals closed the book on one of the city's highest-profile investigations. It began in 1986 with a 17-month undercover FBI sting operation. The sting was revealed in a flurry of subpoenas -- and publicity -- in May 1987 that heralded the broadest federal probe of the D.C. government to date.

That probe, a two-year grand jury investigation into the contracting process, resulted in the May 1989 indictment of Rivers and Clyburn, close friends themselves and both close to Mayor Marion Barry.

Barry issued a statement yesterday contending that the U.S. Attorney's Office "has waged a long and underhanded campaign to discredit the minority business community in Washington. They have failed; the jury showed that Washingtonians are too sophisticated to be fooled by the thinly veiled attempts by the U.S. government to gut the Barry administration's successful partnership with the minority business community."

An unemotional Clyburn said that what began as a "sincere effort" by federal authorities changed along the way and "lost objectivity."

Clyburn contended that there were "racial underpinnings to the case," as authorities put "too much emphasis on minority contractors, unfairly so." That, plus the perception that he had a "close relationship with the mayor," Clyburn said, led authorities to continue with a probe that "all things being equal would have stopped sooner."

"The government went too far," Clyburn, 47, said. "It just should have stopped." Clyburn said the jury's verdict, after 16 weeks of trial and more than five days of deliberation, was "pure vindication."

U.S. Attorney Jay B. Stephens issued a statement saying, "We cannot avoid our responsibility to pursue allegations of criminal conduct simply because the outcome may be unfavorable."

Stephens said that the government presented its evidence, and that the jury "has rendered its judgment . . . . We accept their verdict knowing that we have fulfilled our obligation to present fairly the complex pattern of evidence of the offenses charged."

The prosecution had accused Rivers, the former head of the Department of Human Services, and Clyburn of working together over a four-year period to manipulate the agency's contracting process to benefit them and their friends.

The alleged scheme involved stacking contract evaluation panels, leaking confidential bid information and corrupting other D.C. officials.

The taxpayers were the losers, the government charged, as about $2 million in city contracts went to a private circle of friends -- in some cases unqualified to do the jobs -- instead of to the most-qualified bidders.

Clyburn still faces federal charges in two other cases. In a case scheduled for trial Sept. 12, he and James E. Baugh, a former official at the U.S. Department of Housing and Urban Development, are charged with bribery and conspiracy in an alleged scheme involving federal HUD contracts.

In another case, Clyburn and Porter L. Bankhead, a friend of Clyburn's, are charged with conspiracy in an alleged scheme in which Clyburn was to help Bankhead get a city contract in exchange for a kickback.

"One down, and two to go," said Clyburn, minutes after the verdict was announced in the courtroom of U.S. District Judge Joyce Hens Green, who presided at the trial.

Clyburn showed no emotion as the jurors answered "not guilty" to the reading of four counts against him: one of conspiracy, two of bribery and one of aiding and abetting Rivers in receiving a bribe. "Just my way," Clyburn told a reporter later. "It was what I expected."

Throughout the long trial, defense attorneys said Rivers was simply trying to help minority firms, and Clyburn was doing smart business networking.

Rivers's attorney, Francis Carter, gently hinted at a racial motive in the machinations around one of the alleged contract schemes in the trial. Assistant U.S. Attorney Rhonda Fields countered in her closing argument that the defense was trying to inject questions of race into the case. "David Rivers would have you believe this is all a black-white thing," said Fields, who, along with fellow prosecutor Thomas Motley, is black. " . . . But that's not what this case is about."

Fields and Motley portrayed Clyburn as a savvy puppeteer who liberally used his influence with Rivers's agency.

Clyburn's attorney, Thomas Dyson, told the jurors that his client was using his government contacts as "people in Washington, D.C., do all the time." Clyburn's only sin, Dyson asserted, was "puffing," bragging about his connections.

The defense said the most relevant issue was what the government failed to show: that payoffs had been made. "There's no money," Dyson told the jury.

It was indeed an unusual public corruption case, with no allegations of payoffs, lavish trips or secret bank accounts. Instead, the case hinged on Rivers's alleged acceptance of promises of future payments, of free meals and hotel rooms, and of three pair of cowboy boots.

The trial was touched by developments in the drug and perjury trial of Barry in the same courthouse. After testimony in the Barry case that Rivers had used cocaine with the mayor, the Rivers-Clyburn jury was sequestered. Rivers has not been charged with any drug violations.

From the moment it was revealed on May 22, 1987, the Rivers-Clyburn probe has been intertwined with broader investigations into the Barry administration. On that day, FBI agents, armed with subpoenas, descended on Clyburn's office and on the District Building office of Rivers, then the D.C. secretary.

FBI agents also searched the home of Karen K. Johnson, a former D.C. employee who had a personal relationship with the mayor and who pleaded guilty to cocaine charges in 1984. Federal prosecutors were probing allegations that Clyburn and others made more than $20,000 in payments to Johnson.

Reporters swarmed, and as the summer waned, the story only grew hotter. The relationship between Barry and then-U.S. Attorney Joseph E. diGenova, strained already, only grew more rocky over the many investigations involving city officials.

By August, two grand juries were in session, and the broad scope of the investigation was clear. Although the mayor was not its formal target, no one disputed that he was at the center of the storm.

In September, the FBI searched the D.C. police department's 4th District vice office, in the culmination of yet another federal probe, this time of allegations that some officers were skimming drugs and money. The FBI action opened a rift between Maurice T. Turner Jr., the police chief at the time, and federal authorities.

During the Rivers-Clyburn trial, Barry's name came up occasionally, mostly in a social context. In one conversation, taped by the FBI, Clyburn said he nicknamed the mayor "Supreme."

In the 1986 probe, undercover FBI agent Leonard Carroll, posing as an Atlanta businessman seeking D.C. contracts, met Clyburn and sought his advice. In one taped conversation, Clyburn told Carroll that maneuvering to help firms obtain city contracts was "maybe a little unethical, but nothing illegal . . . . We're just trying to help some minorities get some business."