A 4-year-old boy died after choking on a large grape while eating lunch at a Tokyo kindergarten and police have launched a probe into the accident, investigators said Tuesday.

The boy suddenly stood up with a pained expression on his face while eating grapes of a variety that measures about 3 centimeters in diameter, provided as part of lunch by the kindergarten in Hachioji, western Tokyo, at around 1:30 p.m. on Monday.

Kindergarten staff tried but failed to make him spit out the grape and the boy was taken to a hospital where he was pronounced dead, the investigators said.

According to the investigators, three grapes were served to each student. A peeled grape was found in the throat of the boy at the hospital.

About 25 kindergartners ate the school lunch in a room with two staff members assisting. "We feel deeply sorry for losing an invaluable life," the head of the kindergarten said.

The police are questioning staff members on a voluntary basis over a possible case of professional negligence resulting in death.