Jurors considering the complicated car theft ring case of a D.C. police officer said last night that they did not believe his accuser, a second officer who plays a pivotal role in a continuing federal grand jury investigation into alleged corruption among some D.C. vice officers.

Still, they found Thurlow K. Norris guilty on four counts because of extensive evidence presented in the complex, four-week trial, a majority of Prince George's County Circuit Court jurors said last night.

The trial produced two sharply conflicting portraits of 38-year-old Detective Curtis Arnold, a key witness in the case, who figures prominently in the separate federal investigation.

One portrait, offered by the prosecution, suggested that Arnold is a zealous cop enforcing the ethics of his fellow officers, while the other, offered by the defense, depicted a money-hungry man who occasionally straddled the line of police ethics.

The negative portrait was apparently accepted by a majority of the jurors. "I think that overall Curtis Arnold was lying," said juror Diana Brown, 42.

Most of the jurors said it appeared that Arnold had a vendetta against Norris.

"It was very difficult to believe {Arnold} . . . . He had his finger in so many pies. It seemed like a vendetta," said 56-year-old Giselle Van Ness.

Said foreman Dwayne Day, 25, "The entire jury found Curtis Arnold not to be a credible witness."

Arnold, a Pennsylvania-born, 17-year member of the force assigned to the city's 4th District, surfaced two months ago as a figure in the police corruption probe when he delivered to federal and police officials tape-recorded conversations of drug dealers alleging that some 4th District vice officers had warned them in advance of Operation Caribbean Cruise, a massive police drug raid in 1986.

In early 1986, Arnold tape-recorded admitted cocaine addict Joseph A. Young Jr., who implicated 4th District Officer Norris, 42, in the stolen car ring. The allegation sparked the FBI investigation that resulted in Norris' trial.

A startling twist early in the trial shifted much of the focus from defendant Norris to accuser Arnold and presented the tape-wielding Arnold in a different light.

On Oct. 19, Young, the prosecution's chief witness, reversed earlier testimony and said that a vengeful Arnold had coerced him into making the allegations against Norris and plied him with liquor before the taping session.

Young testified that Arnold threatened to prosecute Young for stealing about $800 from Arnold's used car business, Diamond Auto Sales, if Young did not repeat information Arnold had given him on Norris to law enforcement officers from Virginia, the District and the FBI -- and, eventually, to a federal grand jury.

On the witness stand Oct. 23, Arnold denied he had coerced Young, saying that he had learned only the week before that Young would recant his earlier tape-recorded statements. Adding that he was worried about Young, Arnold cast his eyes toward Norris in the courtroom and said of Young, "I didn't know if he was threatened or what."

Arnold testified also that he had turned over his tape-recording of Young to the D.C. police Internal Affairs Division. Within a day, Arnold said in court, Norris confronted him and asked him why he had accused Norris with the tape. Arnold testified that he was surprised that Norris had learned of the tape, because such internal affairs evidence is confidential.

Young, who contended that Arnold sought revenge against Norris because of a dispute, added another allegation in his testimony: that Arnold on at least 10 occasions had supplied Young with cocaine that the detective had seized in drug raids but did not turn in to the police department.

The allegation mirrored those under investigation by federal authorities, who are now presenting evidence to the federal grand jury that several 4th District vice officers -- other than Arnold -- kept drugs and money seized in raids. Arnold denied Young's drug allegations on the stand.

As the trial progressed, other similarities between the two federal investigations emerged in Judge David Gray Ross' old-fashioned, third-floor courtroom in Upper Marlboro.

Arnold testified that he taped Young because he believed that Young would refuse to tell his story to any 4th District police officer but Arnold. Police officials have said that they authorized Arnold's unusual investigation of the alleged Caribbean Cruise leaks after Arnold convinced them that the drug dealers would only talk to Arnold, and not other 4th District officers.

After Young's surprise testimony, Fred R. Joseph, Norris' defense attorney, took direct aim on Arnold's credibility, suggesting that Arnold was a dishonest police officer who, among other things, actively pursued business interests that may have conflicted with his police duties.

Arnold testified that his wife once owned a half-interest in Charlie's Bar, which is at 7703 Georgia Ave. NW in the 4th District, where the detective is assigned. Arnold testified that he sometimes ran bar-related errands for his wife using an alias.

Arnold testified that he believed that the D.C. police had regulations prohibiting officers from having ownership interest in bars. "I wasn't there," he testified. "And you can't run a bar unless you're there."

Police regulations, in effect at the time, prohibit police officers from employment in, or ownership of, businesses that the department regulates or enforces the regulations of -- except as uniformed security guards. Bars are covered by such regulations, which do not address ownership by relatives.

Arnold testified that to supplement his $37,000-a-year police income, he and his wife have been involved in various other investments and side businesses, including part ownership of three race horses and running a cleaning service for homes and offices.

Arnold testified, in addition, that he bought into a Northwest Washington used car business in which Norris previously had been a partner with another police officer.

Arnold's honesty also came under scrutiny on the question of whether he tried to use the media in the Norris case. According to Young's testimony, Arnold was disgruntled that the FBI investigation of Norris was proceeding slowly and so arranged for Young to tell a reporter for a television station about the car ring.

In his testimony, Arnold said he took a call from the reporter but said that he generally did not talk to reporters and did not understand the workings of the news media.

Norris' defense hinged as well on Arnold's credibility in the federal probe of alleged police corruption, with testimony elicited to show that the 17-year officer was locked in a conflict with Police Chief Maurice T. Turner Jr.

That dispute centers on whether Arnold, after playing the Caribbean Cruise tapes for the chief, was ordered to turn them over to the Internal Affairs Division, which investigates police corruption. Turner has said that Arnold was ordered to do so, but disobeyed.

On the witness stand, Arnold disputed Turner's version of events.

"I received no direct request to take any tape to IAD. The tapes that was made was played for the chief of police in his office," Arnold said, in his first public statement about the Caribbean Cruise tapes. "I'm denying that I was ever instructed to take any tape to Internal Affairs Division."

Each day during the trial, a representative of internal affairs sat on one of five burlap-covered, wooden spectator pews in the courtroom, scribbling notes. At recesses, the official raced to the telephone to report to police headquarters on the trial.

The Prince George's County jury, meanwhile, was not apprised of Arnold's specific role in the federal investigation of police corruption in the District, and it appeared slightly curious about the increased media interest each time the detective took the stand.

In the end, it was left to the 12 jurors to sort out the contradictory portraits of Arnold.

"I have every confidence that you can read right through Curtis Arnold," defense attorney Joseph told the jury. "Old Curtis Arnold is going to get everyone."

"Mr. Arnold is a good cop," countered Jeffrey L. Harding, the prosecutor. "His name has been dragged through the mud because of one man, Mr. Norris."

Staff writer Keith Harriston contributed to this report.