A 12-year-old Rockville girl was charged as a juvenile yesterday in the fatal stabbing of her 15-year-old brother, who was killed after the siblings argued over whose turn it was to use the telephone, law enforcement sources said.

The girl stabbed her brother, identified by neighbors as Rashad Mobley, in the heart with a steak knife Thursday night, the sources said. They said the slaying capped a day of bickering between the brother and sister, who had stayed home from school because they said they were not feeling well. Montgomery County police charged the girl with second-degree murder.

"After fighting all day long, it came down to who would get to use the phone first," said a source familiar with the investigation, who spoke on condition of anonymity. The slaying occurred just before 6 p.m. in a two-story, brick-and-wood townhouse on Moore Drive in the Lincoln Terrace public housing complex.

There is no provision in Maryland law for a 12-year-old suspect to be prosecuted as an adult for second-degree murder. If convicted in a juvenile court, the girl could be confined for no longer than nine years, until she turned 21. Because of her age, The Washington Post is not using her name.

At a closed court hearing yesterday, Montgomery County Circuit Court Associate Judge Katherine D. Savage ordered that the girl be held at the county's juvenile detention center until her trial, scheduled for April 30.

The girl was led in handcuffs into Courtroom 18 by a sheriff's deputy shortly before the 1:30 p.m. hearing. She wore blue-and-black flannel pajama pants, a yellow fleece pullover and tan Timberland boots, her ankles shackled. Her shoulder-length curly black hair was pulled back, and she held her head down, her brown eyes expressionless and hollow.

State's Attorney Douglas F. Gansler said after the hearing that prosecutors had few options available when deciding how best to handle the "extremely rare" case. He said that if the girl had been at least 16 years old, his office would have been required by law to charge her as an adult. If she had been 14 or 15, Gansler would have had the discretion to pursue the case in either juvenile or adult court. But because she is younger than 14 -- she turned 12 in January -- there was no choice but juvenile court, Gansler said.

"We would've had the authority to file a petition to move the case to adult status if the charge had been first-degree murder, if we had an idea that there was premeditation," he said. "We do not feel, given what we have now, that there was premeditation."

However, he said, the fatal stabbing was "clearly not a mistake or an accident. Clearly, she intended the consequences of her actions."

The unusual case could eventually pose a dilemma for prosecutors. "We have a mother who lost one of her children and another child who has been accused of causing that death," Gansler said. "The difficulty for us here, too, is if she's found involved, what do you do about it?" Recommending confinement, he said, would be tantamount to asking a judge to inflict more agony on the mother.

The 35-year-old single mother, wearing a black hat and jeans and carrying a black tote bag, attended yesterday's hearing with several relatives but declined to talk about the case. The accused child's attorney, Dennis J. Kelly, an assistant public defender, did not respond to a telephone message seeking a comment.

Rashad was one of three siblings, including a 10-year-old girl who was home when the stabbing occurred, police said. They said the mother was not in the house at the time of the stabbing. The children were being watched by a 33-year-old uncle, according to neighbors in Lincoln Terrace, a complex of 65 townhouses. They said they could not recall the family causing a disturbance in the neighborhood.

Marteese Brown, 36, who lives two doors from the family, said she entered the house after hearing a commotion and saw the steak knife lying by the stairs. She said she followed a trail of blood up the stairs and saw Rashad in a bedroom.

"When I went upstairs, I found the body on the floor," she said, adding that Rashad's 12-year-old sister was screaming that she had "stabbed him by accident."

Carin Saez, who calls herself the "neighborhood mother" and lives across Moore Drive from the Mobleys, said she also went to the house immediately after the stabbing and described the scene as chaotic, with the 10-year-old also crying.

The 12-year-old girl "knew what happened, but she didn't know the depth of what happened," Saez said. "She just said, 'I didn't mean to do it.' " But then the girl said, "I didn't do it," according to Saez.

When Saez arrived at the house, rescue workers were performing CPR on the boy. He was pronounced dead at Shady Grove Adventist Hospital.

Another neighbor, Jacinta Jones, 27, said the children's mother "worked a lot" and "kept to herself." But Jones and others said the three children were a familiar sight in the neighborhood, playing in the streets and on the small patch of grass in front of the family's home.

Petal Fanfair, 14, who lives around the corner from the Mobleys, said she and the 12-year-old girl are friends and that the girl always was well-dressed. "I didn't know she could do something like this," Petal said. "How could she have a knife in her hand, fighting?" She said she also knew the dead boy, saying he enjoyed playing basketball in the neighborhood.

"He was funny," Petal said. "He used to play around a lot."

Staff writer David A. Fahrenthold contributed to this report.

Jacinta Jones lives in the same complex as the boy who was stabbed and says the children were a familiar sight in the neighborhood.