Pennsylvania authorities are hunting for the person who murdered a couple and their teenage son in their suburban home in the middle of the night.

Authorities are pleading with the public to help them track down the assailant and asked the FBI for help. They are urging area residents to keep their doors and windows locked.

"The suspect is out in the community somewhere," Manheim Township police Chief Neil J. Harkins said Monday evening.

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," 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. Harkins described the commotion Maggie heard as a "scuffling sound."

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.

It's unclear whether Thomas Haines was already dead at that point, or just unconscious.

The Lancaster Intelligencer Journal reported that Maggie then ran to a neighbor's house for help, and never saw her brother.

Click here to read the Intelligencer Journal story

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

Officers arrived about five 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 an upstairs hallway.

Autopsies revealed that they died of stab wounds. "Violent death for all three," Lancaster County Coroner Gary Kirchner said, according to the Lancaster Intelligencer Journal.

He said no weapon was recovered.

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

Investigators said they are researching the histories and lifestyles of all four family members in hopes of finding an event or person of interest related to the slayings, reported the Intelligencer Journal.

Police canines and helicopters combed the area near the slayings Monday, the newspaper reported. One of the bloodhounds trailed a scent early in the day, but investigators would not say if the trail was left by the killer.

Meanwhile, Manheim Township residents remain on edge.

Board of commissioners president Carol Simpson told the Intelligencer Journal the killing is "beyond horrible" and "unimaginable."

"There is shock and pain. Everybody in the township is on edge," Simpson said. "The heinousness of the crime is unexplainable.

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.

The Associated Press contributed to this report.