Detectives investigating Wednesday night's stabbing death of 16-year-old Gateway High School cheerleader Demi Cuccia were able to talk to the suspect, 19-year-old John Mullarkey, at Allegheny General Hospital, where he remained in critical condition from what were believed to be self-inflicted stab wounds. Detectives said the conversation led them to believe he killed Demi and then attempted suicide.

The motive isn't clear, but the two had a rocky relationship. Based on statements from Demi's neighbors and mutual friends at Gateway High School, where Mr. Mullarkey had graduated in the spring and Demi would have been a junior, Mr. Mullarkey was a possessive boyfriend who controlled her every action. Some said she feared him.

"The rumors are that it was an on-again, off-again relationship," said Megan Houghton, a neighbor of the Cuccias. "She was trying to break it off."

Jodi Cuccia, a personal trainer who runs Pinnacle Fitness in Monroeville with her fiance, had taken her son, Jake, to register for classes at Community College of Allegheny County. It's not clear where the younger sister was.

No one was home at the house yesterday, where Jake's pickup sat in the driveway and a memorial candle burned outside the front door.

Ms. Houghton said police and the media had inundated the neighborhood since the crime and everyone now wanted to be left alone.

"The girls are grieving for their best friend," she said. "The whole community's grieving."

Neighbors in Mr. Mullarkey's nearby neighborhood, where he lived with his parents and two siblings, gathered at his house on Laura Lee Drive yesterday but would not comment.

School officials would not allow reporters to approach students or parents on the Gateway campus. Grief counselors were talking to families throughout the day.

Gateway Superintendent Dr. Cleveland Steward Jr. released a statement saying, "I speak for the entire Gateway community when I say how shocked and deeply saddened we are by this terrible tragedy."

On the Facebook networking Web site, Demi's friends posted condolences in a special memorial, R.I.P. Demi Cuccia, set up by 2007 Gateway graduate Tony Ross.