A D.C. police officer was convicted last night by a Prince George's County jury of three counts of theft and one of conspiracy stemming from the officer's involvement in what prosecutors described as an interstate car theft ring.

The Circuit Court jury deliberated 13 hours before reaching the verdict against Thurlow K. Norris, 42, a 19-year member of the department who had been assigned to the 4th District in Northwest Washington before being suspended without pay last November. The jurors found Norris not guilty of seven other counts.

The verdict came in the fourth week of the trial -- one of the longest criminal trials ever held in the county -- in which Assistant State's Attorney Jeffrey L. Harding portrayed Norris as the mastermind in an East Coast car theft ring operating between New York and the Washington area.

One witness, Raphael Ortiz of New Jersey, testified that he drove cars from New York to the Washington area and that Norris had once referred to one of them -- a Toyota Celica -- as being "hot," or stolen. The jury convicted Norris of the counts related to that vehicle.

But several of the jurors said they believed startling testimony from a key prosecution witness, who recanted an earlier statement implicating Norris in the car theft ring. That witness, Joseph A. Young Jr., testified that his statements implicating Norris were made under coercion from another D.C. police detective.

Young, who testified in the second day of Norris' trial, 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 had given statements to 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 an organized ring that stole cars in New York, bought similar models from salvage yards and switched identification numbers on the vehicles.

But in his trial testimony, Young said that Arnold coerced him into making those statements by threatening to press charges against him for stealing about $800 from Arnold's used car dealership, Diamond Auto Sales in Northwest Washington.

Arnold denied Young's allegations on the witness stand, but a majority of the jurors said last night that they did not believe Arnold.

"The jury didn't think Curtis Arnold was credible at all," said Dwayne Day, 25, the jury foreman. "We believed {Young's} testimony here in court."

Prosecutor Harding, who urged the jury in closing arguments to remember that Norris and not Arnold was on trial, said the state provided enough evidence without Young's testimony to prove Norris' involvement in the ring.

Fred R. Joseph, Norris' attorney, said the conviction likely will be appealed because of an apparent contradiction in the verdict. The jury found Norris guilty of possessing and selling one stolen car but also found him not guilty of selling the same car with a defaced serial number.

Joseph had based much of the defense on attacking the credibility of Arnold, and he suggested after the verdict that the Internal Affairs Division of the D.C. police force will look at Arnold's actions in this case.

Norris will be sentenced Dec. 23. The maximum sentences for the offenses of which he was convicted would total 60 years if imposed consecutively, and he could be fined up to $4,000.

Staff writer Victoria Churchville contributed to this report.