The officer suspected "some kids" were just playing their music too loud, but what he found was no run-of-the-mill summer student party: he later told colleagues it was like a scene from Baghdad.
Gangs of students, high on drink and drugs, were hurling rocks into the road. Cars were swerving to avoid them. Debris littered the road. It was mayhem.
Fearing reprisals, neighbours who had called the police refused to give their names. The police officer called for back-up as the youths began throwing rocks at the windows of houses on the neat, tree-lined streets.
Eventually, after reinforcements had arrived, the students calmed down. Police made only one arrest: the person they held responsible for the party and the disorder.
Her name? Amanda Knox, or, as she prefers to be known, Foxy Knoxy.
According to the court records, the first officer on the scene, Patrolman Bender, said the only excuse that Knox gave for her behaviour was that she was moving abroad.
Yet this was not a simple case of mischief at a party getting out of hand: according to police sources, it is rare for a party incident involving a 20 year-old to end up in court unless police believe a serious crime has been committed.
Those present that night say the scenes on the street were as nothing compared to what was going on inside Knox's rented house, less than a mile from where she studied English, German and Italian amid the calm, regulated life at one of America's greatest universities.
It was, according to one party guest, "bedlam, with drink, drugs and bodies everywhere.
"Some people were naked inside the bedrooms.
"There were people draped over each other.
"I've been to a lot of student parties in my time, but I've never been to a party like that.
"Everyone just wanted to get drunk, get high and get laid. There was also a lot of violence because everyone was so pumped up."
Four months ago in Seattle, Knox was fined $269 (£135) at the Municipal Court after the incident - Crime No: 071830624 - and warned that any repeat of her behaviour would lead to much stiffer punishment.