Ronald Ellis pleaded guilty yesterday to killing six persons, including his wife and two daughters, in a mass murder last May described by police as one of the most violent crimes in Prince George's County history.

Ellis' guilty plea to five counts of murder and one of voluntary manslaughter came at a tense hearing in county Circuit Court after his attorney told the judge, "Mr. Ellis is not afraid of the death penalty," but wanted to spare his family the trauma of a trial.

Ellis, 34, received the prosecution's assurance that it will not seek the death penalty against him in exchange for his withdrawing his original plea of innocent by reason of insanity. Instead, the prosecution will ask for five consecutive life terms, a sentence that would keep Ellis imprisoned for at least 60 years. Judge Jacob Levin will impose sentence Thursday.

The two-hour hearing yesterday in the packed courtroom climaxed a series of events that began last May 2 when the bodies of Ellis' wife, Ingrid, their daughters, Tammy, 12, and Monica, 4, and three others were found in the couple's Camp Springs home. Ingrid Ellis had been shot with a handgun downstairs, and the others were killed with shotgun blasts in Monica Ellis' upstairs bedroom.

Ron Ellis fled after the killings, allegedly abducting an acquaintance and her young son in suburban Virginia and then traveling to Chicago. Ellis soon telephoned his father, who advised him to "Get in touch with the FBI." Four days later, Ellis unexpectedly gave himself up in a midnight surrender at the FBI's main headquarters in Washington.

Stunned friends and relatives said that Ron Ellis, a printer, and Ingrid, a D.C. police officer, had been having domestic problems and that Ingrid was leaving and taking the children. The couple's other daughter, Tracey, 15, had left home prior to May 2 and was at the movies with a friend at the time of the slayings.

Tracey Ellis, now living with a relative, was not in the courtroom yesterday, but Ellis' father, brothers and sisters were there, sitting tight-lipped and solemn, some close to tears. During one strained moment in the proceedings, as the prosecution began to detail the way in which Ellis' wife and daughters were slain, it appeared that Ellis would change his mind and withdraw his guilty plea.

That point came when Prince George's County State's Attorney Arthur A. Marshall Jr. described a "wound to the head by a shotgun" suffered by Monica Ellis. Defense attorney R. Kenneth Mundy jumped to his feet. "May I object, your honor," the angry Mundy asserted.

Then Mundy and two other defense attorneys bent over and huddled with their client in the hushed courtroom, Ellis whispering fiercely to them, slashing the air with his hand and shaking his head negatively.

Finally, Mundy stood and addressed the judge. "One reason Mr. Ellis was prepared to plead guilty was to spare his family. He has a daughter left alive out of this tragedy," Mundy said. "Mr. Ellis is not afraid of the death penalty. One of his primary reasons for pleading guilty was to avoid this type of gruesome testimony. If we must hear it . . . we will hear it at trial," Mundy said, referring to some of the prosecution's evidence as "this stuff. . . this garbage."

When Mundy finished, Judge Levin ordered Ellis to rise, and asked him simply whether he shot each of the victims. Ellis stood, his fingers planted rigidly on the defense table, and weakly answered "Yes," after each inquiry. Finally the judge asked him in the required legal language, "Are you pleading guilty. . . because you are in fact guilty?"

Ellis answered, "As I told you before, your honor, I don't know if I shot anybody."

Nevertheless, Levin found that Ellis had voluntarily changed his plea to guilty. Earlier in the hearing, the defendant had answered affirmatively when asked if he had shot each of the victims. He also acknowledged that he understood his constitutional right to trial and was voluntarily relinquishing it.

Levin, however, ordered Ellis to be examined at Clifton T. Perkins Hospital by state psychiatrists to determine if he is legally competent to understand the proceedings and assist his lawyers. Levin still must formally accept Ellis' plea Thursday. If Ellis is found incompetent, the plea would not be accepted, and Ellis would receive psychiatric treatment until he became competent to stand trial.

Ellis' plea to voluntary manslaughter was for the slaying of Ingrid Ellis. Voluntary manslaughter does not carry the legal element of an intentional, purposeful killing, as does murder, and it is often used as a charge in killings growing out of domestic arguments.

The hearing yesterday provided some new details about the slayings and the crime scene. Marshall stated that the shotgun used on the five victims in the bedroom was fired 15 times, meaning that it was reloaded at least three times during the slayings, according to Marshall.

But the hearing did little to unravel the mystery of precisely why six people, including three of Ingrid Ellis' acquaintances -- Janet Jackson, her son Tyrone and Sherry Robinson -- were killed on May 2.

Ellis, looking grim in a gray sweatshirt and green pants, said little during the hearing and nothing on his way out of court. After the hearing, Mundy, said, "Perhaps lurking in the back of his mind is that he could someday get out and atone for this in some way to someone."