She was barely breathing when the graveyard shift officers from the 43rd Precinct found her in her apartment in a Bronx public housing complex early yesterday morning.

The officers had responded to 911 calls reporting a struggle in apartment 8-J at the gloomy Bronx River Houses, and, when they opened the door, they discovered Ophelia Torres, 16, with 15 to 20 stab wounds to her chest and back.

Nearby was her older cousin, Johnny Torres, 20, with fewer gashes, to his chest, back, and neck, but fatal ones.

Ms. Torres was rushed to Jacobi Medical Center and was in critical condition last night, police said. Her cousin was taken to the medical examiner’s office for an autopsy.

While murders and violent assaults are not new crimes to the officers and detectives of the 43rd Precinct – there were 15 murders and 497 felony assaults last year, making it one of the top 10 areas in the city for violent crimes – one compelling aspect of the double stabbing is the possible impetus of the crime: a puppy dog.

Perhaps more compelling is that when the accused stabber, Ms. Torres’s boyfriend, Samuel Encarnacion, 18, first made contact with police, he led them directly to the bodies.

Yesterday, Mr. Encarnacion, who lived near the Bronx River Houses, a public complex that houses more than 3,000 low-income tenants in more than 1,200 apartments in nine buildings, was in custody and awaiting a handful of charges: murder in the second degree, manslaughter, attempted murder, assault, and criminal possession of a weapon – a kitchen knife.

According to police, the events unfolded around 2:45 a.m. when Mr. Encarnacion came to see his girlfriend, Ms. Torres. She was in the apartment with her cousin, Johnny Torres, and they had begun to train a newborn black-and-white spotted pit bull of mixed pedigree.

In some neighborhoods, police said residents often train their dogs for protection. To do so, they only let the dog’s primary owners come in contact with the dog during the early stages of training – the idea being to keep the dogs untrusting and wary of as many people as possible.

Once in the apartment, police detectives believe that Mr. Encarnacion asked the Torres cousins if he could pet the dog, and they objected. He asked again. They objected. Then police said a shouting match, then a fistfight, erupted between Mr. Encarnacion and Johnny Torres.

Mr. Encarnacion then hurried to the kitchen, police said, where he grabbed a knife and began the attacks, first on his girlfriend’s cousin, then on his girlfriend.

With his clothes sullied and spattered with blood, police said Mr. Encarnacion stripped himself of the garments and stuffed them and the murder weapon down a garbage shoot. He then took refuge in a friend’s apartment on the 11th floor of the complex, and told the friend his girlfriend had been stabbed by someone else and to call 911, police said. He then tried to flee, and was met by police officers in the lobby.

He told the officers his girlfriend had been stabbed and escorted them to the crime scene. They noticed Mr. Encarnacion sweating, breathing abnormally, and making incriminated statements.

He was placed under arrest. If convicted of the charges, Mr. Encarnacion stands to face a prison sentence of 25 years to life. A court-appointed lawyer had yet to be assigned as of last night.