FLORENCE, Italy -- An autopsy has determined an American woman whose naked body was found last week in her Florence apartment was strangled with an object such as a rope or cord, an Italian prosecutor said Tuesday.

Prosecutor Giuseppe Creazzo cautioned more lab results are needed to pinpoint the time of Ashley Olsen's death.

The 35-year-old had been living in Florence for about three years. Autopsy findings so far indicate she died in a 36-hour-period between Friday morning and early Saturday afternoon.

The corpse, with bruises and scratches on the neck, was discovered after her Italian boyfriend expressed alarm he hadn't heard from her in a few days and asked the landlady to open the apartment door.

Olsen, originally from Summer Haven, Florida, was last seen by friends early Friday at a popular Florence nightclub.

It could be weeks before results of analyses of body tissue and fluid are ready to help experts narrow the time frame when she was strangled, Creazzo said.

Laboratory results also will help determine whether Olsen had been sexually assaulted, Creazzo said, adding there were no external signs of sexual attack.

There were reportedly no signs of struggle against whoever killed her.

Creazzo said at this point, investigators have no suspects.

Italian news reports have said police haven't found Olsen's cell phone.

Olsen's friends, in a letter appearing in an English-language biweekly in Florence, said they are hoping whoever killed her will be brought to justice.

"While we mourn her passing, we place our trust fully in the Italian authorities to investigate this tragic episode," read some of the sentiments in the letter in The Florentine. "We wait to hear what they discover, and pray that justice will be swiftly served to whoever his responsible."

The friends described Olsen as a "rare and kindhearted spirit."

"We have seen the messages of love pour out from her hometown of Florida and we know that, across two countries, people who knew and loved her are in shock," they wrote.

Corriere della Sera newspaper quoted the boyfriend as saying the couple had quarreled over a minor matter three days before her corpse was found, and that he tried to call her but that Olsen didn't answer her phone.

It was unclear when the body might be released for burial, as prosecutors won't give the go-ahead until they are sure more forensic medical testing isn't needed for the coroner's report.

Olsen's father teaches in Florence, and her social circles included the art community in the Tuscan capital, renowned for its Renaissance architecture and masterpiece-rich museums and churches.