"Jose told her that he knew where the family was," a family member said yesterday.
The aunt, Isabel Martinez, entered the apartment alone and found the bodies.
The dead were identified by family members as Carmen Hernandez, 23, who was four months pregnant; her husband, Catalino Hernandez Jr., 44, and their children, Jonathan, 9 months, and Frankie, 6.
Police said Jose Hernandez, an 11th-grade student at Murrell Dobbins Area Vocational Technical School, had not been seen since he opened the front door for Martinez.
"We don't know what his role in the crime is," said Capt. Robert J. Grasso. "We are trying to locate him to question him to find out what happened here."
Grasso said the four victims apparently had been dead at least two or three days.
"I don't want to describe the scene graphically, because there are things about the crime scene that would appear to be known only to the (killer). That information may be significant in later determining who committed this crime," Grasso said.
Dr. Halbert Fillinger, assistant Philadelphia medical examiner, said last night that autopsies determined that Frankie had died of suffocation and strangulation and that Jonathan had died of a skull fracture after being struck with a blunt object. Their bodies had been wrapped in plastic bags, Fillinger said.
Autopsies on the parents' bodies, conducted last night, showed that Catalino Hernandez had died of head injuries and that Carmen Hernandez had been shot to death, sources said.
Catalino Hernandez Sr., father of the slain man - who was called "Junior" by family and friends - stood outside his slain son's apartment yesterday, his voice choking with emotion.
He said Martinez, his daughter, had gone to Jose's school early yesterday to question her nephew about the whereabouts of his family. "She was concerned because they had disappeared," the elder Hernandez said.
"Jose told her that he knew where the family was."
Jose went with her and opened the door, "but as soon as he opened the apartment, he was away and gone," the grandfather said.
"Isabel entered alone. She started to look inside the house, and when she
went to the bathroom, she saw them," Hernandez said.
Police said the bodies were found under a pile of clothing in the bathtub, which contained no water. The two children were lying underneath their parents, police said.
The killings shocked friends of the family.
"When it came over the news, I just ran out into the street screaming," said Nilda Lopez, 29, a family friend who lives in Kensington. "They had been married about five years. As far as I know, they were a happy couple. And the relationship between Jose and his father was good." She said Carmen Hernandez was about four months pregnant.
Police described Jose Hernandez as 5-foot-5, with a light complexion and wavy black hair. When last seen, he was wearing a blue-and-white jacket and white sneakers. Police said he may be driving a 1979 four-door maroon Honda or a black Chevrolet van.
"These things are not supposed to happen," said Miguel Carmona, 25, a nephew of Catalino Hernandez Jr. "That's why we want to find Jose, to find out what happened."
Carmona said he had telephoned his uncle's apartment for nearly two weeks, and had stopped by on several occasions "to see if someone would answer the door."
"A couple neighbors told me they saw the son come in and pick up a few bags and leave. All the windows in the house were open - which is odd in cold weather," Carmona said.
Yesterday, at Dobbins, a vocational-technical high school at 22d Street and Lehigh Avenue, Jose Hernandez was described as "an average student, not exceptionally above or below average," said acting principal Donald Bakove.
Bakove said Hernandez, who is majoring in auto mechanics, was absent from school yesterday.
When the school let out at 3 p.m. yesterday, several cars containing uniformed and plainclothes police were parked near the entrances. Students stood around in small knots, many wondering about the police presence.