The lifeless body of the 24-year-old was discovered aboard the Regent Seven Seas Voyager on Sunday when ship docked in Australia Sunday morning, ABC reported.
Detective Superintendent James O'Brien said the crime division no longer suspects the incident was a crime and are waiting on the results of an autopsy to determine the cause of death.
"Further toxicology results will be analyzed in due course to determine the exact cause of death," he said.
The night before Kastrinelis was found dead, the singer suffered a head injury for which she received treatment by the ship doctor before going to bed, Cruise Law News reported.
According to a passenger's account, the crew is being held in the ship until the case is resolved.
"It is a shame and you have to feel for the crew because they are the unfortunate ones," said John Harold, a passenger from Los Angeles. "They aren't letting any of them off the cruise ship. They are the ones being held captive," he said.
The singer's family was informed of the incident by the U.S. consulate and arrived in Australia today to meet with her body. The cruise line, payed their respects to them in a statement.
"The Regent Seven Seas Cruises' family is deeply saddened by the unexpected passing of Jackie," the luxury cruise line said in a statement. "Our hearts and prayers go out to her family and loved ones."
Kastrinelis began working as a cruise ship singer in 2011 after graduating college.