A 4-year-old boy has died after choking on a large grape during lunch in 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 as part of the lunch for each kindergarten student, and one 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. The head of the kindergarten told Kyodo News, "We feel deeply sorry for losing an invaluable life."

The police are questioning kindergarten staff members on a voluntary basis, with an eye toward building a case of professional negligence resulting in death.