A key witness in the car theft trial of D.C. police officer Thurlow K. Norris reversed himself in testimony before a Prince George's Circuit Court jury yesterday, saying that statements he had made implicating Norris in a car theft ring were made under coercion from another D.C. police detective.

Joseph A. Young Jr. identified the detective as Curtis Arnold, a key figure in a federal probe into alleged leaks of information before a 1986 drug raid in the District.

Young's startling testimony, which could be admission to the crimes of perjury and giving false statements to a federal officer, came in the second day of Norris' trial, in which prosecutors allege that Norris was the key player in an East Coast car theft ring operating between New York and the Washington area.

Young, of Northwest Washington, had told law enforcement officers from Prince George's County, Virginia and the FBI, as well as a federal grand jury in the District of Columbia, that Norris and several other men were part of a ring that stole cars in New York, bought similar models from salvage yards in the Washington area and switched serial numbers on the vehicles.

Assistant State's Attorney Jeffrey L. Harding said that Young's grand jury testimony and statements to police "completely laid out the ring's operation."

But yesterday, Young testified that Arnold coerced him into making those statements by threatening to press charges against him for stealing $700 from Arnold's used car dealership, Diamond Auto Sales in Northwest Washington. Young stole the money to support a cocaine habit, he testified.

"Mr. Arnold made up the story," Young said during court testimony. "I repeated it. I can't say whether {Norris} is guilty or innocent. I just know that I don't know anything about a car theft ring."

Young testified that Arnold "wanted to get Norris" because Norris had sold Arnold a British sports car for Arnold's sister, but the car caught fire because of a defect and was destroyed on the way to her in Pennsylvania. Young testified that Norris refused to return Arnold's money for the car and that Arnold believed Norris was selling used cars for Arnold without paying Arnold.

"Arnold said T.K. {Norris} laughed at him," Young testified.

Arnold also provided Young, on 10 to 12 occasions, with small amounts of cocaine that Arnold said he had seized from drug suspects but not turned in to the department, Young testified.

Arnold also arranged for Young to tell the story about the car theft ring to Mark Feldstein, a reporter for Channel 9, Young said. "Arnold said he wanted me to meet with Feldstein because the FBI was dragging their butts in the investigation, and he was scared they were going to lose the case," Young testified.

Arnold could not be reached for comment. Harding, the assistant state's attorney, said Arnold may testify today in the case.

Norris, 42, an 18-year member of the D.C. police force, has been on suspension without pay from the department since November. A sergeant from the department's internal affairs division watched yesterday's proceedings as part of the administrative investigation into the allegations against Norris. The sergeant would not comment about Young's allegations against Arnold.

Arnold, a 17-year member of the D.C. department, is a key figure in a D.C. federal grand jury investigation of whether members of the 4th District vice squad tipped-off suspects to Operation Caribbean Cruise, a drug crackdown that fell far short of expectations.

After the operation failed, Arnold investigated the alleged leaks and secretly recorded conversations with drug dealers who said D.C. officers told them about the drug sweep before it was staged. D.C. police are investigating why Arnold's investigation was not turned over to the Internal Affairs Division.