It was the perfect crime. So who made the fatal error?

It was the sensation of the year in Italy. Two law researchers, students of Nietzsche, were accused of executing a motiveless murder. But something about the conviction has caused unease.

At 3pm on a steamy June day, the high-security courtroom near Rome's Olympic stadium is packed. After a 13-month trial and nearly 30 hours deliberation, the jury is about to release its verdict and the tension is palpable. The accused, Giovanni Scattone 31, and Salvatore Ferraro, 32, junior lecturers in the legal philosophy department of Rome's La Sapienza University, chat nervously with their lawyers as the jury president steps forward. The court finds Scattone guilty of firing the shot that killed law student Marta Russo and sentences him to seven years jail. Ferraro is given a four-year term for aiding and abetting.

The two young men, pale and disbelieving, are escorted swiftly out of court by the police. At 8pm Scattone and Ferraro grant an exclusive interview to state television RAI for the main evening news bulletin. The interview, for which they were allegedly paid 100m lire (pounds 30,000) provokes rowdy protests from opposition channels. Critics say it's offensive to use taxpayers money to finance convicted killers. Both young men say they are the victims of a clamourous miscarriage of justice.

At 11pm the two young academics have parted company; Scattone is relaxing in his local pizzeria with friends while Salvatore Ferraro takes a stroll in Piazza Navona after a family dinner. Under Italy's Byzantine legal system, a sentence is only enforced once the defendants have exercised their right to appeal to two higher courts. For now the two convicted killers are free. After two years of preventative custody and house arrest they are keen to stretch their legs.

This sequence of events is nothing compared to the twists and turns that have marked the two years since law student Marta Russo was gunned down just before noon at La Sapienza.

The scene of the crime, a narrow lane between the law and statistics faculties, was busy that day. Yet not one person saw who fired the fatal shot and from where, nor was anyone spotted fleeing the area and no weapon was discovered. One minute Marta and her friend Jolanda Ricci were chatting about their exams, the next minute, Marta slumped to the ground unconscious. Only as blood began to seep from her friend's head did Jolanda realise Marta had been shot. She never regained consciousness and died four days later.

No amount of delving into her private life turned up a single reason why anyone should want to kill Marta. She was an averagely popular, averagely academic student who dreamed of becoming a successful lawyer. She lived with her father, a physical education teacher, housewife mother and student sister in a middle-class suburb of Rome. There were no disgruntled ex- boyfriends, extreme political views or drug problems.

Italians, accustomed to Mafia slayings and political killings, were deeply unsettled by the random nature of the crime. The terror of a serial killer on campus spread. The police complained that within the law faculty they were getting scarse cooperation.

Senior figures at the university feared for the reputation of La Sapienza. Despite having little to go on, the authorities assured the public that Marta's assassins would be caught. An autopsy showed that the bullet had entered Marta's head from behind and on the left. That corresponded with the direction of the toilets on the ground floor, frequented by hundreds of people per day. But then came the breakthrough, forensic tests showed traces of gunpowder on the sill of a window on the second floor, a reading room in the legal philosophy department. The circle tightened around the 25 or so people who often used the room to consult textbooks or use computers.

A month after Marta's murder, police arrested two junior lecturers. One, Giovanni Scattone, was accused of firing the fatal shot from the reading room. The other, Salvatore Ferraro, was charged with being an accomplice. Neither had a convincing alibi. The problem was there was no weapon, and no apparent motive. The two accused did not personally know Marta Russo. But the absence of a reason for the killing began to seem like a motive in itself. The investigators argued that the two young men, both from comfortable middle-class homes and with good career prospects, had tried to commit the "perfect crime": a crime impossible to prove because of the lack of any motive to link them with the dead woman.

The prosecution said the two, who taught philosophy of law and were described as academically brilliant, were swayed by Nietzsche's theories on man and superman, and that the killing of a 22-year-old law student they didn't know was an intellectual dare. The prosecution's depiction of the two accused as arrogant was reinforced by their behaviour in court. Giovanni Scattone, with big blue eyes and a fleshy baby face appeared indifferent, while his more talkative, preppy looking friend Salvatore Ferraro was described by the press as arrogant.

It was never going to be a linear legal case, but few were prepared for the dramatic twists in the inquiry since the pair were arrested in June 1997.

The Marta Russo case depended almost exclusively on eyewitnesses, in particular Gabriella Alletto, a 45-year-old secretary who testified that she saw Scattone at the window of the reading room with a gun in his hand and Ferraro putting his hands to his head in despair. The presence of the two junior lecturers was confirmed by two other witnesses. However some weeks after the crime, one said her recollections were "subliminal", another's account was punctuated with the phrase "don't remember" and the third retracted his statement, claiming he'd been threatened. Much of the case depended on the credibility of Gabriella Alletto who held her own well in face to face encounters with the two young academics. That was until the court was last year shown a video tape from a hidden police camera: in a break during a tough interrogation, Gabriella Alletto is shown despairing and in tears, swearing that she was not in the reading room at the time of the killing.

The video was shot three days before she signed a full declaration saying she had seen Scattone and Ferraro. Several of Ms Alletto's friends, called as defence witnesses, said she had told them she was being pressured to accuse. The magistrates' governing body has opened disciplinary proceedings to see whether the two public prosecutors abused their powers.

Then earlier this year the scientific evidence incriminating Scattone and Ferraro was called into question. A panel of independent experts appointed by the court ruled that the particles found on the reading room window ledge were "probably incompatible with gunpowder".

All that remained was the theory of the perfect crime. As the trial drew to a close, the public was divided between vocal "innocent" and "guilty" camps.

Giovanni Scattone and Salvatore Ferraro were convicted of Marta's killing by an eight-member jury. But in reducing the charge from murder to involuntary manslaughter for Scattone, and aiding and abetting for Ferraro, the jury has thrown out the perfect crime theory. Rather they saw Marta's death as a tragic error from playing with dangerous toys.

Marta Russo's father, Donato, said he had fulfilled his promise to her, that her assassins would be identified but admitted that it hurt to see them walk free pending appeals. "We live in a system where only the accused has his rights guaranteed," added Marta's mother Aureliana.

Scattone's defence lawyer, Manfredo Rossi, slammed the verdict as "a sordid compromise". Commentators and public figures also expressed perplexity about what looked suspiciously like a compromise, "a Pontius Pilate sentence".

One of the jurors, in an interview, defended their decision. "The fact is that we have not managed to understand the motive of this thing. For murder you need a motive and there wasn't one."

"I did not commit any murder, voluntary or involuntary," said Scattone in his highly paid television interview. "If I had done, I would have said so immediately and avoided a year and a half in jail. It was the first thing they proposed to me after my arrest."

"The investigating magistrates depicted us as monsters," said Ferraro after his sentence. "There are still people who believe in a non-existent diary of mine in which I supposedly wrote `the blonde passes by and off goes her head.'"

The court acquitted other defendants, star witness Gabriella Alletto and usher Francesco Liparota, of aiding and abetting, on the grounds that they had been threatened. The department head, Professor Bruno Romano, charged with obstructing justice, was also cleared, though police accusations that he urged his staff not to cooperate left a long shadow. The impression in the early stages was that academics at one of Italy's most prestigious institutions were more concerned about damage to their career prospects or the university's reputation than finding a murderer. The appeals court hearing is scheduled for this autumn, and meanwhile, the two men convicted of causing Marta Russo's death are free to go to the beach for the summer.