Howard County prosecutors were dealt a potentially crippling blow in a highly publicized murder case yesterday when a second key witness said she would not testify next week against Dimitrios G. (Jimmy) Prassos, 23, accused of murder, robbery and conspiracy in the Aug. 2 slaying of his father, a well-known Ellicott City restaurant owner.

It was the second time in three weeks that a major prosecution witness has indicated a refusal to testify at the Prassos trial, scheduled to start Monday.

At a Jan. 13 pretrial hearing, Lisa Williams, 19, refused to answer questions about statements prosecutors said she had made to police implicating Prassos and his codefendant, William H. Ray Jr., 30, in the robbery and slaying at the Pirate's Cove Restaurant.

Yesterday, a second witness, Kathie McMillen, 27, said she would not testify against Prassos or Ray because police and prosecutors have failed to place her and her family in a witness protection program.

Police and prosecutors declined to comment on McMillen's statements.

"I've lost my house, my career and 9-year-old son," McMillen said in a telephone interview. McMillen, a drug and alcohol abuse counselor, said she and her fiance have received harassing phone calls in the last few weeks.

"My life is more important than theirs," McMillen said, alluding to Prassos and Ray.

"It's been very, very frightening. I'm not going to take that chance," she said.

Assistant State's Attorney Richard P. O'Connor declined to comment on the latest development in the case, which has been stymied by a dwindling list of credible witnesses.

O'Connor said he and defense attorneys cannot talk about the case because of an order imposed by Circuit Court Judge Thomas J. Nissel Jr.

Asked what impact McMillen's refusal to testify may have on the prosecution's case, O'Connor said "we'll find out on Monday" when the trial is scheduled to begin.

Last week, O'Connor filed a memorandum asking the judge to allow the prosecution to use hearsay evidence at the Prassos trial because of reluctant or uncooperative witnesses.

Remaining on the witness list is a former cellmate of Ray's at the Maryland State Penitentiary in Jessup, according to court documents filed by prosecutors last week.

McMillen said yesterday she told police that Ray, a former patient in the detoxication unit at St. Joseph's Hospital in Towson where McMillen previously worked, told her about a robbery.

McMillen said she initially decided to cooperate with police after being promised anonymity through the Metro Crime Stoppers Program.