Wednesday night's stabbing death of Monroeville cheerleader Demi Cuccia, apparently by a man described by mutual friends as a possessive boyfriend, has parents wondering how to keep their teens out of abusive and potentially lethal relationships.

Therapists yesterday cited several warning signs in stormy adolescent friendships ripe for catastrophe -- red flags such as a boyfriend or girlfriend who is controlling, overly demanding, demeaning and jealous.

But experts warn parents before they try to break up such a relationship, be aware that adolescent romance is blind and obsessive by nature.

"It is a crazy time," said Dr. Paul Friday, chief of clinical psychology at UPMC Shadyside. "It is puppy love. It is the only obsessive compulsive disorder that society approves of. Nothing else matters. You can't eat. You can't do anything. You just have to think about you and your boyfriend or girlfriend.

"When something like the stabbing of a teenager happens, we are all aghast. What can we do? What are the warning signs? Unfortunately, the warning signs are everywhere -- spending too much time together, possessiveness."

His comments came as Allegheny County homicide detectives and Monroeville police were still piecing together the case.

But investigators believe John Mullarkey, 19, plunged a knife into the chest of his girlfriend, Demi Cuccia, in her home shortly after 7 p.m. and then slashed his own throat in an attempt to kill himself.

Officers called to the scene found the teens on the pavement outside the Cuccia family's townhouse on Elliott Road, where Demi, who had just turned 16, lived with her mother, Jodi Cuccia, her 13-year-old sister, Sage, and her older brother, Jake.

Police said the stabbing occurred in the living room and that the two teens staggered outside before collapsing. Officers discovered Mr. Mullarkey about 25 feet from Demi's body. They found a folding knife in the bloody living room.

Both were taken to Forbes Regional Hospital, where Demi was pronounced dead at 7:06.

Mr. Mullarkey, who nearly died from his wounds, was flown to Allegheny General Hospital. He remained in critical condition yesterday.

One problem in trying to decipher whether relationships could end in violence is that there may be multiple warning signs, but then the relationship breaks off "with no harm done,'' Dr. Friday said. In other cases, "you might only have two signs and the guy flips out."

Despite the ambiguity, therapists yesterday said there are certain traits that are at least indicators of something potentially harmful.

These would include a person who monopolizes your time, insists you check in several times a day, is jealous, degrades you and blames you for his or her bad behavior, said Michael Barfield, executive director of Outreach Teen and Family Services in Mt. Lebanon.

Threats -- or, of course, physical abuse -- are glaring warning signs of something terribly amiss. "When people make threats, then the line is definitely crossed," said Dr. Oscar Bukstein, associate professor of psychiatry at Western Psychiatric Institute and Clinic.

But often a controlling mate knows how to camouflage bad behavior in front of parents and transform himself into Mr. Charm. "He is so mannerly," Mr. Barfield said. "Johnny is so nice. When Johnny is away, he is a different person. You say, 'How could I have not seen this?' You are not necessarily going to see it."

Even if you know a boyfriend or girlfriend is bad for your child, telling them to stay away from a romantic interest can backfire.

"Often adolescents will do just the opposite," Dr. Bukstein said. "Parents have to take care they are not overly critical. You have to let adolescents air their doubts without being judgmental."

Mr. Barfield recommends that instead of condemning a relationship and getting into a power struggle with your child, try saying, "Help me understand your relationship with John or Sue. What is good about it? What is bad about it?"

But once a love interest physically abuses your child, call the police, he says. "They can be mad at you all they want. They are your child."

John Amato, director of training and consultation services for the nonprofit counseling center FamilyLinks in Shadyside, says the best advice a parent can tell a teenager is, "You have a right to be respected and not to be treated less than a person. You don't have to settle for anything less than being respected."

Start having those discussions with your child early -- before they start dating, the experts said.

"If parents read about a tragic story in Monroeville and they say, 'What can I do for my teenage kids,' probably it is too late if you haven't been talking to them about it already," said Dr. Friday.

When this kind of tragedy strikes, there is often a tendency to second guess loved ones about what they could have done differently. That was the case almost 18 years ago of another teenage girl, Karen Hurwitz. She was stabbed and strangled in the back yard of her Squirrel Hill home by a platonic friend, Michale Anderson.

"It is easy to place blame," said Robert Mendelson, the Pittsburgh author of an upcoming book on the Hurwitz case, "The Chase for Beauty."

"It is easy to say in hindsight, 'We saw it coming. This is why it won't happen to our child,' " Mr. Mendelson said. "In Karen's case, she was simply being a good friend" to a troubled man.

"Her parents trusted her and the decisions she made. And by all accounts she made good decisions," he said. "There is no way anyone could see this happening. If there are lessons to be learned, the lesson is not to point fingers at those who remain."