A 30-year-old convicted murderer who apparently left with family members visiting him Sunday at the District's Lorton Reformatory remained at large yesterday, officials said.

D.C. Department of Corrections officials were still trying yesterday to determine exactly how J.D. Wheeler of Southeast Washington escaped. Wheeler had been confined in Youth Center 1 at Lorton since August, officials said, and had been scheduled to be sentenced for an armed robbery conviction today.

"This is unheard of, somebody just walking out of here," John F. Henderson, acting administrator of Lorton's youth facilities, said yesterday. "He was to be sentenced Wednesday, so I guess he figured he had nothing to lose. Somehow, he discovered a flaw in our system."

Corrections officials said Wheeler was the first inmate to escape from a medium- or maximum-security facility in two years. They said it was unlikely that Wheeler was armed, but said he could be dangerous because of his record of violent crimes.

Henderson said yesterday that he had formed a committee to begin investigating how the escape occurred, and added that he had tightened procedures to prevent any other escapes at Lorton, which is in southern Fairfax County. The investigating committee will review the center's security policies as well as the physical layout of the facility, which is surrounded by a fence and guard towers.

In October 1986, Wheeler began serving a 10-year sentence for second-degree murder in Maryland, officials said, and also had a pending charge for armed robbery in the District.

Wheeler was taken to the youth center in August because the murder conviction and the armed robbery charge violated the conditions of his parole, officials said. In 1976, under the federal Youth Corrections Act, Wheeler was sentenced to serve an undetermined amount of time for robbery and armed robbery convictions.

Between June 1979 and April 1986, he had been paroled three times. Henderson said Wheeler's parole conditions dictated that he be returned to the center, which is a medium-security facility that houses 406 inmates.

"We judge each of them to see if they present any immediate escape threat," Henderson said. "And his behavior up until Sunday exhibited none of that."

Officials said they believe that Wheeler escaped from the center between 11:30 a.m. and noon Sunday. Henderson said that, at 11:25 a.m., Wheeler's mother, his girlfriend, his brother and two children arrived to visit him.

Just before noon, a corrections officer found a pair of pants and a coat, the kind issued to prisoners, in a women's bathroom in the center. An emergency count revealed that Wheeler had escaped, Henderson said.

Corrections officers count everyone who enters and leaves the center daily, he said, but often cannot be sure if an inmate is missing until visiting hours end.

"To count everyone every time someone leaves would stop movement all day long," Henderson said. "All our officers would be doing nothing but counting all day."