While the accused sat impassively in a Baltimore County courtroom here today, a Prince George's County policeman detailed the defendant's account of the night that ended in the violent death of Stephanie Ann Roper at the end of a lonely southern Maryland road April 3.

The signed statement of Jack Ronald Jones, 26, given on Easter Sunday, highlighted the second day of his trial for the alleged abduction, rape and murder of the 22-year-old Frostburg State senior who was home that spring weekend with her friends and family in Prince George's County.

"I must have been stupid to ever have gotten myself involved in something like this," Cpl. Frank Gale quoted Jones as saying.

Jones, according to Gale, said he struck Roper with a chain harder than he intended when she tried to escape and set her afire only after feeling no pulse.

Jones' statement, admitted into evidence over the objections of his court-appointed lawyers, told how he and Jerry Lee Beatty, 17, had found Roper and her disabled car by a wooded country road in the early morning hours of April 3. Beatty, scheduled to be tried separately, is expected to testify against Jones this week.

Roper had returned from an evening at a Foggy Bottom disco with Lisa Thomas, her college roommate also home for the weekend, to Thomas' parents' house in Clinton. Roper had started home when her car ran off the road.

Jones, a pale, thin man, said he and Beatty helped push Roper's car off the road and offered further assistance, according to Gale.

Roper then asked for a ride back to her girlfriend's house, Gale said Jones told him. But instead of letting her off, the two men passed the house as Beatty, armed with a rifle, rode with Roper in the back seat.

"The girl was afraid of Mr. Beatty. She didn't seem afraid of me," Gale quoted Jones' saying. "She was crying a lot in the back seat." While Beatty was raping Roper in the car, Jones said, "She was crying for Lisa to help her, for God to help her."

After they arrived at an abandoned house in the Oakville section of St. Mary's County, near the Patuxent River, both men raped her, Jones said. When she tried to escape, he said he hit her over the head with a chain and then told Beatty to "hold a gun on her" while he went to get gasoline. He intended, he told the policeman, to burn her clothes and any identification cards in her possession and then let her go.

When he returned with the gasoline, however, she had no pulse, so he set her afire. The two men then dragged the victim by her feet to a nearby swamp, according to the statement Jones gave police.

The trial was disrupted earlier in the day as C. Clarke Raley, the St. Mary's County prosecutor trying the case, asked that Roper's grandparents and siblings leave the courtroom prior to testimony from the medical examiner's office about Roper's mutilated body.

Roper's maternal grandmother left the courtroom sobbing and Dr. Ann Dixon, an assistant state medical examiner, testified that the immediate cause of Roper's death was a single gunshot wound to the forehead. In his statement to police, Jones said he fired a gun twice, once inside the car and a second time in the air "to scare the girl." His statement made no mention of shooting her.

Dixon also testified that an analysis of Roper's blood showed she had been drinking that evening but not excessively.