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A tumultuous teenage relationship ended tragically late Wednesday when, according to police, an 18-year-old stabbed his cheerleader girlfriend to death a day after her 16th birthday.

Moments later, John Mullarkey Jr. came out of Demi Cuccia's home and slit his own throat, police and witnesses said. He remained in critical condition Thursday, though police described his prognosis as poor.

The two-year relationship was well-known in the high school, where Cuccia was a cheerleader and Mullarkey had just graduated. Friends referred to the relationship as "delicate" and "on and off."

Donna Walko, 48, a neighbor, described Mullarkey as possessive and jealous.

"I talked to Demi plenty of times. She was sitting in my living room, I begged her to get away from him," said Walko, whose two teenage daughters were fellow cheerleaders and good friends with Cuccia.

Cuccia's mother forbade her to see him, said Walko, still wearing the same denim shorts and sweat shirt she had been wearing Wednesday night when her daughters called to say their friend had been stabbed.

Although the relationship was rocky, the bloody outcome shocked many who knew the pair. They said Mullarkey was funny and not violent, Cuccia friendly and popular.

Monroeville, a middle-class suburb about 15 miles east of Pittsburgh, is a quiet place with large malls, townhouses and planned communities.

"A lot of people didn't sleep last night because you don't think it would happen in Monroeville," said Kaitlin Nemec, 16, a friend of Cuccia's.

But national statistics show teenage relationships, fraught with high emotion, often turn violent.

According to a 2000 report by the U.S. Department of Justice, women ages 16 to 24 suffer the highest per capita rate of dating violence. One in three high school students have been or will be in an abusive relationship and most teen dating violence takes place in the home of one of the partners, the report found.

Monroeville Police Chief George Polnar said Mullarkey has not yet been charged with a crime because his prognosis was poor. Police are not aware of the two having a violent relationship.

"Generally, at that age, it doesn't graduate to this level," Polnar said.

On Thursday, neighbors at the secluded row of townhouses where Cuccia lived milled around outside. Moving from house to house, they chatted about the killing, comforting one another and discussing the chain of events.

Cuccia's mother was out of the house when Mullarkey came over to visit, Walko said. He had only been there for about a half-hour when the next-door neighbor heard screaming from the Cuccia house and went to knock on the door, she said.

"Demi came crawling out to the neighbor and said, 'He stabbed me,' and then he came out and slit his throat," Walko said.

The neighbor, struggling to stop Cuccia's bleeding, asked Walko's daughters, who had heard the commotion and come outside, to call 911. Then they called their mother, who was at a parents meeting at the high school.

"They were just screaming, 'Demi's been stabbed. Demi's been stabbed,"' Walko said, nearly breaking down in tears.

Fellow students set up a page on Facebook.com titled "R.I.P. Demi Cuccia." Within hours, friends and classmates had written their memories, thoughts and feelings.

Jon Nemec, Kaitlin's 18-year-old brother, graduated with Mullarkey. The two have been together since grade school. They were on the wrestling and baseball team together.

"He was funny. He was fun to hang out with," Nemec said. "I never thought he'd do it, but their relationship wasn't the greatest."

Kaitlin Nemec chose to focus on Cuccia. After reading through dozens of poems on the Internet, she chose one about a "little angel" to post on the Facebook page.

"She was quiet, but she had a spontaneous side. She really was always happy," Nemec said. "She would befriend anyone."