ROME (AP) _ A fencing champion in her youth, Marta Russo was just one of 41,000 students studying law at the University of Rome.

Then a bullet from nowhere struck her down as she strolled on campus with a friend.

After a month of nationwide speculation, police arrested two assistant law professors and another university employee on Sunday, charging them with voluntary homicide in the May 9 shooting. A prominent law professor has also been taken into custody for allegedly hindering the investigation.

The killing shocked many in Italy, where campus crime is rare, and was closely followed by the national media.

Premier Romano Prodi and thousands of students went to the 22-year-old woman's funeral. In a ceremony attended by President Oscar Luigi Scalfaro, Russo also was awarded her degree posthumously.

Police identified the suspects as assistant professors Giovanni Scattone, 29, and Salvatore Ferraro, 30, and university worker Francesco Liparota, 29. They also were charged with illegal weapons possession.

They face life in prison if convicted.

Though the motive for the killing remains unclear, one scenario holds that Russo was the victim of a deadly game in which the shooter took aim at her and pulled the trigger without knowing the .22-caliber pistol was loaded, according to police.

Said a police official who spoke on condition of anonymity: ``Maybe they thought the gun was not loaded, and said, `Let's shoot that one _ Pow!' ''

Police have not ruled out that the shooter was aiming at another target, possibly political science professor Cesare Marongiu, who was near the path taken by the bullet. Marongiu told the ANSA news agency he did not know the arrested men.

Scattone, a former paramilitary police officer, allegedly fired the gun, which has not been found, police said. Like the victim, he was described by friends and neighbors as a hard-working, athletic, honest young person from a middle-class Rome family.

However, the three men were known to be gun lovers, part of a circle of staff and employees in the philosophy of law department who were interested in weapons, the police official said.

Two witnesses came forward and described seeing the three suspects in the room during and just after the shot was fired.

Scores of investigators worked on the case. Forensic evidence led them to a window at department's offices where they believe the shot was fired.

After questioning staff, police arrested department chairman Bruno Romano on Thursday and charged him with impeding their investigation.

They allege that Romano tried to prevent employees from providing information to keep the department's name from being tarnished.