Nancy and Richard Langert of Winnetka, Ill., a prosperous Chicago suburb, were an affable, hardworking couple whose lives seemed filled with promise. Nancy, 25, was a corporate lawyer's daughter from Winnetka, who sang in the church choir and performed in community theater. Richard, 30, a burly former Chicago Catholic high school athlete, delighted in hosting parties for family and friends. Both had good jobs at Gloria Jean's Coffee Bean Corp., a chain of gourmet coffee retailers based in Arlington Heights, Ill. Richard supervised production; Nancy worked in the franchise sales department. After three years of marriage, they were expecting their first baby and preparing to move into a new house closer to their work. "We have everything going for us," Nancy told friends in January. "This is going to be our year."

But when they returned home from a family dinner celebrating Nancy's father's birthday on a clear moonlit Saturday night last April, an intruder surprised them. The next day the Langerts failed to show up at church or answer their phone. At 4 P.M. Nancy's father went to the town house and discovered their bodies lying faceup in the basement. Richard had been handcuffed and shot in the back of the head. Nancy had been shot in the elbow, side and stomach. With her finger, she had tried to scrawl a message in blood. At first, rumors circulated that the letters spelled IRA. Later, police concluded that the message was a heart next to the letter U.

Though the intruder had rifled Nancy's purse and torn through the boxes the Langerts had packed for their move, nothing appeared to have been taken. The $500 Nancy had from a paycheck cashed the previous day was strewn on the living room floor, leading police to rule out burglary. They followed hunches that went nowhere: They hunted in vain for clues of marital infidelities, unpaid debts and cocaine smuggling at the coffee company warehouse. One theory was that the murders might have been committed by a vengeful client of Nancy's father. Another was that Irish terrorists had retaliated against Nancy's sister, an attorney who monitored human rights abuses in Northern Ireland, a supposition the family considers preposterous. "What was so outrageous about the investigation was that these kids were as straight-arrow as they come," says a family friend. "All the wild speculation only added to the grief and suffering of everyone who loved them."

It seemed for a time that the killer might never be found. For six months the FBI and police from seven Chicago suburbs had run a maze of dead-end trails trying to solve the shocking murders. The killings had stunned a community still dazed by the tragic shooting of six elementary school children two years earlier.

The one place lawmen apparently did not pay enough attention to was the town of Winnetka itself. But at the community's highly regarded New Trier Township High School, senior David Biro, 17, a lanky honor student known for his biting sarcasm and perverse humor, mockingly bragged to classmates that he had shot the Langerts. Accustomed to Biro's frequent claims that he was a hired assassin, drug dealer and street-gang member, most of his fellow students did not take him seriously. Finally, however, one of Biro's schoolmates decided that there was more to the boy's claims than mere braggadocio. Early last month he told Winnetka police what Biro had been saying. Investigators listened closely, since Biro had mentioned details of the murders that had never been publicly released.

It wasn't the first time Biro had caught the eye of police. According to friends—all of Biro's acquaintances cited in this article have insisted on anonymity since they are potential court witnesses—he was admitted to Charter Barclay, a psychiatric hospital in Chicago, three years ago after reportedly trying to poison his parents, brother and sister with tainted milk. Since then he had been stopped by police for a series of minor infractions. When the high school informant told police of Biro's claims of involvement in the Langert killings, investigators recalled that Biro had been spotted, wearing black clothing, near the murder scene on the night of the slayings.

On Oct. 5, the day after police learned of his boasting, Biro was taken into custody for questioning as he left his parents' three-story stucco home. After police searches of his bedroom uncovered items, including a glass cutter, handcuffs and a .357 Magnum revolver, that had been stolen from the office of Biro's former lawyer, the teenager was charged with first-degree murder of Richard Langert and his pregnant wife, Nancy, and the intentional homicide of their unborn child. A .357 Magnum can fire the type of .38-caliber bullets used in the murders, and while law enforcement officials have revealed few details of their investigation, they say they are confident that the .357 Magnum and other items taken from Biro will link him to the murders. Biro is being held at the Cook County jail without bail, awaiting his Nov. 21 arraignment.

Biro's arrest has bewildered the community and left everyone, including the police, wondering why a child of privilege might have committed such a crime. Drawings and writings recovered from his room have led police to consult with ritual-crime experts to determine whether Biro may have dabbled in destructive occult beliefs. His closest friends wonder if Biro may have been "pushed or dared" into committing a crime as a street-gang initiation—or whether he might have killed the Langerts at the behest of someone else.

Some classmates say Biro was driven, as one acquaintance put it, "to test his limits." A red notebook, found in his room by police, contained underlined news articles about the murders, including a magazine story in which an attorney for Nancy Langert's sister observed, "I bet if they ever do catch the culprit it will turn out to be some local teenager." Says one girl: "If [Biro] did do it, it's because he wanted to commit the perfect crime. Dave viewed everything as an intellectual challenge, and he may have been too smart for his own good."

One question remains: Why the Langerts? Though Biro's parents were casually acquainted with Nancy Langert's parents, police admit they are puzzled about the nature of the relationship—if any—between Biro and his alleged victims. Although the teenager accompanied his mother to the Langerts' funeral, police say there is no evidence he knew them before the night that their paths may have fatally crossed.

While Nancy and Richard Langert appeared to be living their dream, David Biro was decidedly less content. The son of a successful public relations executive, David, the youngest of three children, was an intelligent, inquisitive youth who suffered the painful rejection of classmates. In eighth grade, he resorted to wearing a fake cast on his arm to ward off the attacks of other students who chased him after school. He, in turn, began harassing neighborhood children with urine-filled balloons and BB guns. "They thought he was weird, so he kept up the image," says a former classmate.

After his hospital stay, Biro admitted to friends he felt even more like an outcast. As a defense against stinging remarks that he was "psycho" and "crazy," he cultivated the role of a tough guy. He studied martial arts, had his arms tattooed, flashed street-gang hand signs and dressed in a long black coat that he believed made him look like a gangster.

Professing dislike for the "preppie" crowd that dominated New Trier, he sought companionship from an older group of friends in the city, including some he claimed to have met while he was hospitalized. Yet he also ran for class president (he lost), joined the high school cross-country team and began scouting colleges. "No one could ever figure Dave out," says a fellow student. "Everyone knew who he was, but no one really knew him."

Perhaps Biro didn't either. Observes a friend: "He was always looking for answers for the meaning of things, like who God is and who the devil is." Violence and power seemed to fascinate him. He talked about the "insane genius" of Charles Manson and Hitler, and he was intrigued by movie villains and assassins, often mimicking the characters. One of his favorites was the psychopathic hit man played by actor James Woods in the film Best Seller. Acting the part one day, Biro demonstrated to a companion that he could withstand pain when he burned his palm with a cigarette. Says a friend, "He said he wanted to be the ultimate assassin." When Biro goes to trial, Winnetka may finally learn whether that ambition became more than a fantasy.