Six weeks after he killed a CIA officer in a street crime in Northwest Washington, Ronald T. Stephenson made a mistake.

He suspected that D.C. police had no solid leads in the June 2000 killing of 28-year-old John Muskopf Jr. and got drawn into a conversation about the slaying by an old friend, Dwight Walker, at Walker's apartment in Landover.

"So who hit him?" Walker asked.

"I did," Stephenson said. "But if . . . unless anybody have a . . . videotape saying that I did it. You know what I'm saying?"

As it turned out, Stephenson was providing police with that video.

The conversation was recorded by a small microphone hidden in a couch cushion and a tiny camera hidden inside a clock radio. The grainy tape became a key to securing Stephenson's conviction on felony murder and other charges.

Yesterday, Stephenson, 20, stood expressionless as he was sentenced in D.C. Superior Court to 36 years to life in prison.

Law enforcement officials said that it is extremely rare for D.C. police to have a cooperating witness use a hidden camera to record a statement from a murder suspect.

But Assistant U.S. Attorney Jay I. Bratt said this case could have easily slipped into the abyss of unsolved D.C. homicides if it weren't for Walker -- a 64-year-old cook -- and the compelling firsthand account he got from the suspect.

Faced with the hidden camera footage, Stephenson later gave D.C. police a statement in which he admitted to the shooting. That, too, was videotaped. Jurors saw both tapes at Stephenson's recent trial, despite objections from Stephenson's attorney, Ferris R. Bond, who argued that the secret taping tainted the case.

At yesterday's sentencing, Judge Robert I. Richter said that Muskopf's death -- in an apparent robbery attempt near his home -- caused an "incalculable loss" to his family and country. Muskopf, who joined the CIA in 1997, spoke French and Farsi and specialized in nuclear-related issues. CIA Director George J. Tenet called him a "rising star."

Stephenson, of Landover Hills, was a petty thief. Years ago, Walker befriended him and became "something of an older brother figure," said Bratt, who prosecuted the case.

On June 17, 2000, Stephenson was behind the wheel of a stolen car when he and two friends, Shelton Ford and Darrell Hendy, pulled up alongside three men walking near 10th and S streets NW, about 2:30 a.m. to rob them, Bratt said.

Muskopf and two friends were walking to Muskopf's home, just 50 yards away, after visiting two nearby clubs. Muskopf had just moved to the house from Arlington.

After Stephenson pulled over, Ford wrapped a shirt around his face, jumped out of the car with a .38-caliber revolver, grabbed one of Muskopf's friends and demanded money from all three men before firing two shots at the ground, Bratt said.

Muskopf told his friends, "Don't worry, guys. Those are only blanks." Then, his friends said, he told Ford: "Get out of my neighborhood."

That was when Stephenson pointed a gun out of the driver's-side window and fired, striking Muskopf in the neck, Bratt said.

Two weeks later, Stephenson first told Walker about the killing, the prosecutor said.

"He was willing to turn a blind eye on some things. But not murder," Bratt said.

Detectives asked Walker if he'd be willing to record Stephenson talking about the killing. Walker suggested using the hidden camera in his apartment, Bratt said.

On July 28, 2000, Walker called Stephenson to set up the sting. About 8 p.m., Stephenson arrived at the apartment, wearing long shorts and a skullcap. He took a seat on the couch, but away from the camera, prompting Walker to ask him to move. Stephenson smoked a cigarette and drank from a bottle of beer as Walker's parakeet chirped in the background.

"You ever killed a person before?" Walker, out of view of the camera, asked.

"That was my first time." Stephenson replied.

"How do it feel when you kill somebody?"

"I could tell you the truth, I felt bad."

The statements led to Stephenson's arrest on Aug. 10, 2000. Walker, meanwhile, entered the federal witness protection program. Stephenson's alleged accomplices, Ford and Hendy, were not charged.

Just before Stephenson was sentenced, Muskopf's father, John Muskopf Sr., of Roanoke, addressed the court.

" I have to say that as a Christian, I must forgive, but I cannot forget," he said. "I want [Stephenson] to understand that the life he took meant more to us than anything in the world. It's devastated my life."

CIA officer John Muskopf Jr., 28, was killed in a street robbery. His death was called an "incalculable loss."The grainy video by a man who had befriended suspect Ronald T. Stephenson helped secure his conviction on felony murder and other charges.