FREMONT, Calif. (AP) _ ''Dear Diary: ... I killed my little sister.''

That's part of the sole diary entry of a 14-year-old Fremont girl accused of using a pillow to smother her 4-year-old sister, who died under mysterious circumstances.

The teen-ager appeared in juvenile court Wednesday for arraignment on a murder charge. She didn't enter a plea but has previously denied causing her sister's death.

Juvenile Court Referee Ted Johnson ordered her held in a juvenile detention facility until her next court appearance Jan. 6. Both parents were present during the hearing, but refused to speak with reporters.

The names of the victim and her sister were withheld because of their age.

An investigation began Aug. 18 when the girls' parents found their asthmatic younger daughter dead in her bed.

The Alameda County Coroner's office listed the cause of death as undetermined.

But natural causes didn't explain the trauma of the death and the police investigation ''focused on unnatural all the way,'' Sgt. Rich Phillips said Wednesday.

Last weekend, authorities said, the 14-year-old ran away from home and her parents opened the diary they had given her for Christmas, finding the following entry:

''Dear Diary: Well, it's the New Year so I'm going to start telling you everything dearest to me. I have to get something off my back. I killed my little sister.

''I went into her room and got her and took her into my room and told her I loved her and covered her mouth and suffocated her. You're the first person I've told. Well, I feel better.''

The entry was dated Jan. 1, 1993.

The parents called police after discussing the entry with the family's pastor.

Police arrested the girl early Sunday at her boyfriend's house. But she wasn't charged until Tuesday, after the coroner's office went over the case again and determined that, despite her asthma, the 4-year-old had no pre- existing medical condition that would have caused her to stop breathing.

She initially refused to acknowledge to police that she wrote the entry. Later, according to officers, she admitted writing the statements but insisted she did so only to gain attention.

The diary statement was significant, but was just one link in an investigation that started after the 4-year-old was found dead, Phillips said.

''We focused on her mental stability from the start,'' he said. ''But we needed something more than the fact that she had a propensity for doing such an act.''

Phillips said the teen-ager had a long history of psychological problems, but wouldn't go into details.

''We are not experts on giving psychiatric evaluations,'' he said.

The parents told police that although the 4-year-old suffered from asthma, she was fine when they put her in her bed. The child took medication and used a special machine to make sure the medicine went deep into her lungs.