A 4-month-old baby boy has been found dead with a fractured skull at an unlicensed nursery in Hiratsuka, Kanagawa Prefecture, prompting police to launch an investigation.

A male nursery official called for an ambulance at around 4:30 a.m. Sunday, saying the baby boy was not breathing, police said Wednesday.

The baby was rushed to hospital, but doctors pronounced him dead on arrival. An autopsy showed that he had a fractured skull and died of bleeding on the brain. No other external injuries were found.

Police said an acquaintance of the baby's mother dropped off the baby at the facility around midnight on Saturday.

Unlicensed nurseries are not illegal in Japan, but rather do not meet certain criteria, such as room space and a playground, to be granted government subsidies.

They generally operate longer hours than licensed nurseries, making it convenient for parents who need to work late at night.

On Monday, the Kanagawa prefectural office conducted an emergency on-site inspection, confirming that there was only one official at the nursery at the time.

Kanagawa's regulations call for at least two or more officials on duty, with the facility instructed to rectify the situation.

According to the prefecture, the nursery had been taking care of four other children from Saturday night to Sunday morning.

Opened in October 1992, the nursery provides a 24-hour service with six staffers and accepts up to 20 children.

A man claiming to be the head of the nursery said on Wednesday, "We have video footage recorded at the time (he died) but the baby boy has not been sleeping face down and our staffer has not dropped him. I don't know why he died."

He added that the nursery will close down until details become clearer.