Police said on Monday they believed a cyanide poisoning incident in western Japan that killed four people was likely an attempted mass slaying.

"We're still investigating all possibilities, including chances of an accident, but it appears the possibility of the case being a murder is strong," a police spokesman said.Four people died and 42 were in hospital with cyanide poisoning on Sunday after eating curry and rice at a summer festival in Wakayama Prefecture, 280 miles west of Tokyo.

The four who died Saturday night and Sunday morning were a 10-year-old boy, a girl aged 16 and two men aged 53 and 64.

Autopsies confirmed cyanide poisoning as the cause of death in at least three of the four cases, police said.

Police said the curry was prepared in two large pots in the garage of a local resident and in a smaller pot at a nearby home and then carried to the site of the festival, several hundred meters away.

Curry in the smaller pot and one of the larger pots was not eaten after people at the festival became ill, but no cyanide was found in these.

But the remaining pot was cleaned after the curry had been dished out, prompting police speculation that its contents had been laced with the poison.

Cyanide was also detected in water in a nearby sewer, apparently after one of the pots was washed out, police said.

The National Police Agency, calling the case an "evil and peculiar crime," on Monday dispatched two senior investigators to the site of the festival in the small town of Wakayama to join local police.