A group of children found a boy's naked body jammed into a suitcase floating in a duck pond at a suburban Sydney park, triggering a murder investigation Thursday and a police warning to parents.

There was widespread outrage at the apparent brutality directed toward the unidentified child, who police said did not match the description of anyone on missing-persons lists, though clues indicated the body had been there for days.

"Put simply, it's one of the most monstrous acts imaginable," New South Wales state Premier Morris Iemma told the state legislature.

A group of about 10 children playing in the park in the Sydney suburb of Rosemeadow pulled the dark, tartan-patterned bag of carry-on luggage size ashore on Wednesday evening and reported their gruesome discovery after opening it.

"We were riding on our scooters when a friend came up and said there was a dead pig down at the duck pond, so we went down to see," one of the children said on Australian television. His name was not given and his face was obscured because of his age.

"Me and my friend got sticks and flipped it over and then we saw that there was feet, so we went up to our houses to call the police," he said.

Homicide Detective Superintendent Geoff Beresford said the dead boy — who was jammed in a fetal position and wrapped in plastic — was believed to be age 4 to 8, revising downward an earlier estimate of 5 to 10.

Beresford said witnesses told investigators that they had seen the bag floating in the pond for at least two days, and that the body was in the early stages of decomposition.

A post-mortem examination began Thursday to try determining who the boy was and how he died. Beresford said it was too early to say if he had been assaulted.

Police divers scoured the pond Thursday and found items including clothes and a knife, though investigators had not yet linked them to the death.

Beresford also said police were looking into reports of a suspicious white van seen recently in the suburb.

"It's certainly being investigated. Obviously I would certainly be wary if I was a parent," Beresford said.