The last two of eight young men convicted by a jury of murdering Catherine Fuller were sentenced to maximum prison terms yesterday after a prosecutor declared in D.C. Superior Court that one of the two is responsible for "another one of Mrs. Fuller's murderers walking the streets today."

Judge Robert M. Scott imposed the sentences of 35 years to life in prison on Clifton E. Yarborough, 17, and Kelvin D. Smith, 20, who were found guilty in December of fatally beating Fuller as they and a group of others attempted to steal her coin purse.

Two defendants were acquitted at that trial, and Assistant U.S. Attorney Jerry S. Goren told the judge yesterday that he still believed that one of the two, Alphonso L. Harris, participated in Fuller's murder. Yarborough, Goren said, could have proven that.

"It is my firm belief that Mr. Harris was there, regardless of the jury's" verdict, Goren said during the lengthy hearing at which he gave one of the first glimpses into the police investigation that led to the largest number of arrests for a single slaying in the District.

Yarborough, Goren said, initially gave police statements implicating a number of persons, including Harris, but those statements could not be used at the trial because Yarborough would not take the stand to testify.

And, Goren said angrily, "There is another of Mrs. Fuller's murderers walking the streets today."

Harris could not be reached for comment, but his lawyer, Michele Roberts, said yesterday, "I don't believe Mr. Goren should be acting as a 13th and dissenting juror."

Roberts said she did not believe that Yarborough's statements would have changed the outcome of the jury's verdict, because Harris' defense, as she explained in her opening trial statement, would have been to show that another person with the nickname "Monk" was responsible. "It's all spilled milk," she said.

Before he was sentenced, Yarborough, the youngest of the defendants, said in a barely audible voice that he "needed psychiatric help" now and that he had lied to police when he told them he had seen what had happened in the alley where Fuller was pummeled to death.

"I be afraid," he said, repudiating his police statements. "I just said anything that I thought would get me out of it."

Goren earlier had outlined pre- viously undisclosed facts about how Yarborough gave police information a few days after the Oct. 1, 1984, murder and that led to the arrest of Harris, which was the first in the case.

When Yarborough was arrested two months later, Goren said, he gave police a videotaped statement in which he again named Harris and several other participants.

At that point, Goren said, Yarborough refused numerous plea bargains offered in exchange for his testimony.

Yarborough's videotaped statement was played to the jury, but it had been severely edited to exclude any mention of other defendants or participants because those named would not have had a chance to cross-examine Yarborough, who did not take the stand.

During the trial, two government witnesses who pleaded guilty to reduced charges in exchange for their testimony named Harris as participating in Fuller's beating. No one else named him as a participant. Jurors later said they refused to convict any of the defendants solely on the testimony of those two government witnesses.

"Mr. Yarborough was the third person to identify Mr. Harris," Goren said. Goren said that other participants identified by Yarborough were never brought to trial because of his refusal to testify.