Police referred six doctors to prosecutors on Wednesday over their alleged negligence resulting in the death of a 2-year-old boy at a Tokyo hospital in 2014.

The anesthesiologists, including a 60-year-old former associate professor at Tokyo Women's Medical University Hospital, are suspected of administering an excessive dose of the sedative propofol after the boy underwent neck surgery on Feb. 18, 2014, according to investigative sources.

The toddler, only identified by his first name Kosuke, was artificially ventilated in an intensive care unit at the hospital in the capital's Shinjuku Ward, the sources said.

Administration of the sedative to children requiring artificial ventilation is prohibited in principle due to the risk of side effects, but doctors are allowed to exercise their discretion.

The doctors allegedly continued to administer propofol even though the boy's cardiogram and urine showed signs of abnormality, the sources said.

The toddler, who died on Feb. 21 that year, received propofol equivalent to 2.7 times the level appropriate for an adult over about 70 hours, according to a report by a third-party investigation panel set up by the hospital.

In June 2014, the hospital announced that propofol had been administered to a total of 63 children aged 14 or younger over the five years through 2013.

It said 12 of them had died but denied that the deaths were caused by the use of the sedative.