Clothier, they say, fell victim to drugged-out acquaintances, who forcibly injected her with some sort of controlled substance.
When the injection killed her, the attackers stuffed Clothier's body in a laundry bag, tying it around her neck and using her yellow turtleneck sweater to cover her head.
Then they left her body in a secluded section of the twisting creek. She lay partially submerged, undetected, for nearly five weeks until the fishermen found her.
Bucks County District Attorney David W. Heckler and Northampton Township Police Chief M. Barry Pilla Jr. revealed the answer to the mystery during a news conference at the police headquarters office yesterday morning.
A 2005 tip from a woman who said she believed she'd owned the laundry bag relaunched the investigation.
The three men involved in Clothier's death are now dead themselves. Police refused to identify them yesterday.
"If any of those men had been alive when police received this information, even if they were clinging to life support in a nursing home, instead of standing before you today, we would be working to gather evidence with which to achieve a murder conviction," Heckler said.
"However, these men are dead and beyond the reach of human justice. Since we cannot charge and prosecute them, they will never have the opportunity to defend themselves, and it accordingly would be wrong to disclose their names."
Still, authorities celebrated the sleuthing that led to the killers' identification, saying that solving the case gives closure to Clothier's lone survivor - her older sister - and the hundreds of law-enforcement officers who tried to find her killers.
Clothier's sister, Susan, would not comment, police said. But others who remember the case eagerly expressed their relief.
William Tomlinson was a young Northampton cop when his bosses assigned him to Clothier's case. Now retired, he returned to the fold for the news conference. "It's a good feeling," Tomlinson said, "just fantastic."
THE GIRL DISAPPEARS
In 1968, Clothier was a 16-year-old junior at Lincoln High School. On the evening of March 9, she told her family she was going to take a bus to visit a boyfriend in Mayfair. She had $1 in her pocket for bus fare.
She never arrived.
Police launched a massive search, after one captain theorized that Clothier "had too fine a background in her school and personal life to be a runaway," according to a newspaper article.
Her parents - father Elmer was a city firefighter, mother Evelyn was a secretary - knew her "as a good, obedient girl who heeded their curfew that kept her close to their neat semidetached home in Crispin Gardens. She was home early each weekday night and was permitted to stay out until midnight on Saturdays," the Daily News reported.
Police broadcast a 13-state alarm. For days, more than 150 people aided by dogs and a helicopter searched 5 square miles of woods in Northeast Philadelphia. Volunteers and police handed out more than 10,000 fliers showing a girl with short brown hair and big brown eyes in their quest to find the girl they called "Candy."
Evelyn Clothier wrote a poem about the search for her daughter that was published in The Northeast Times in 2008 as part of a story marking the 40th anniversary of Clothier's slaying:
"Another night before us, another day has passed,
No laughter in this house now, how long will all this last?
Oh God, where is our Candy?
Her radio's not blaring, her books are all in place,
Her stuffed animals stand ready, to welcome her embrace,
Oh God, where is our Candy? . . . "
SEARCH FOR KILLERS
When Clothier's decomposed body was finally found, it was by accident. The fishermen were enjoying the first day of trout season. They spied the bag lying on a small creek island. They recognized the yellow sweater they'd heard the teenager had been wearing when she disappeared.
Bucks County officials said that whoever dumped the girl there had to be very familiar with the area: "There is no other way the killer could have found the spot," a source said at the time.
The search for Clothier's killers was intense. On the day of her funeral, police were conducting lie-detector tests on people of interest in the case.
The one solid clue investigators had was the sack that had held her body. Some felt it resembled an Army-issue duffel bag. Bucks County officials asked for military help. No luck. Investigators handed out more than 8,000 fliers with a picture of the sack, seeking help. None came.
Clothier's father died six months after his daughter disappeared, his body found in his parked car outside the fire station where he worked. Natural causes, the coroner said. Her mother died in 2007.
The break in the case came in 2005, when NBC-10 featured the cold case on a newscast. A viewer called police to report that she believed the sack the fishermen found Clothier in on April 13, 1968, was her laundry bag, missing since she gave it to her husband shortly after Clothier disappeared.
Investigators restarted their probe, spending months to track down witnesses, many of whom were dead. Yesterday, Pilla, the Northampton chief, gave this account of what they believe happened:
Clothier left her home intending to catch a bus to Mayfair to visit her boyfriend. Instead, she uncharacteristically accepted a ride with an acquaintance, who was in a car with at least one other man.
Instead of taking her to Mayfair, the men took Clothier to a wooded area off Decatur Street in Northeast Philadelphia, popular among teens as a hangout. The men had a history of drug use, and one was known to inject drugs into animals and people, without their consent, retired Northampton Detective Charles Wyant said.
Detectives believe that Clothier died after she was involuntarily injected or given an unknown controlled substance, Pilla said. The men then called a third man to help them dump the corpse off the Chain Bridge on Route 232.
Although Clothier was found wearing only panties, the pathologist found no evidence of sexual assault, Pilla said.
The trio he blamed yesterday for Clothier's fate were among those interviewed decades ago; Pilla didn't know why detectives turned their attention elsewhere.
Although the men were from Northeast Philadelphia, they were familiar with Bucks County and at least one had visited a horse farm and riding stables in Northampton Township, Pilla added.
Heckler said the case proves authorities' unflagging dedication in solving cold cases, no matter how old. He urged citizens with any knowledge of other unsolved murders to call their local police or his office at 215-348-6344.
"There's no statute of limitation on murder," he said, "and we will continue to pursue justice on any case."