Authorities said they have no suspects in the weekend killings of a couple and their teenage son in their suburban home and pleaded with the public Monday to help them solve the crime.

Thomas Alan Haines, 50, Lisa Ann Haines, 47, and their son Kevin, 16, were found stabbed to death Saturday at their house on a leafy street in an upper-middle-class neighborhood of Manheim Township, police said at a news conference.

"Someone knows something about this crime," Chief Neil J. Harkins said.

The couple's daughter Margaret, 20, was home at the time and called 911 from a neighbor's house at 2:24 a.m., police said. She was awakened by a noise and went into her parents' bedroom, Harkins said.

Her father was lying on the bed and her mother, who was sitting on it, quietly asked her daughter to seek assistance, Harkins said.

"Her mother said to her to go get help, leave the house and go get help," the chief said.

Margaret, known as Maggie, did not see a suspect or intruder, he said.

Officers arrived minutes after receiving 911 call and found the three victims already dead inside the two-story stone house. The parents were found in their bedroom, and the son was found in a hallway.

Autopsies revealed that they died of stab wounds, but Harkins would not elaborate. He said no weapon was recovered.

"Who the hell did this?" said Lancaster County coroner Gary Kirchner. "I mean, that neighborhood is a class neighborhood."

Authorities repeated a warning to residents to keep their doors locked. Police found the Haineses' back door open when they responded to the 911 call. Harkins would not say if that was how the killer entered, but he noted there were no signs of forced entry and nothing was taken from the home.

The search warrant for the Haineses' home was sealed because it contains sensitive information that could compromise the investigation, Lancaster County District Attorney Donald Totaro said.

Harkins said there would be more searches of the home over the coming days.

A next-door neighbor, Leonard Witkonis, said his windows were open the night of the killings, but he didn't hear anything.

He described the Haines family as quiet.

"They weren't very sociable," said Witkonis, 68, a retired state worker.

Father and son would sometimes play catch in the backyard, and Thomas Haines jogged through the neighborhood, he said.

At Manheim Township High School, where Kevin Haines was a sophomore and member of the German club and Quiz Bowl team, a moment of silence was observed during morning announcements Monday.

"Everybody is kind of in shock," said Marcie Brody, a district spokeswoman.

Maggie Haines was a 2005 graduate of Manheim Township, according to Brody, and had just completed her sophomore year at Bucknell. She returned home for the summer on Thursday, Harkins said.

Totaro said Maggie Haines has cooperated with investigators, and Harkins said investigators have no reason to believe she was involved in the killings.

Thomas Haines was a salesman at Motion Industries in Lancaster. A man who answered the phone at the industrial supply company on Monday declined comment.

Lisa Haines was a teacher at Lancaster Brethren Preschool, according to Jeffery B. Rill, pastor at the Lancaster Church of the Brethren. Her school had its last day of classes on Friday.