Le Hoang Long, a first-grade student at the Gateway International School in Hanoi’s Cau Giay District, died on his second day to school on Tuesday.

He was brought to the E Hospital, a public hospital in Cau Giay, with no pulse and blood pressure. He had been found unconscious in the school bus at 4:20 p.m. by the school staff.

Long’s family, who only learned of his absence from school in the afternoon, agreed to have an autopsy done to identify the time and cause of death. The boy is his parents' only child.

The results show Long had died before arriving at the hospital, Cau Giay District officials said at a press conference on Wednesday. They did not reveal the cause of death.

The district police were informed of the situation on Tuesday afternoon and they summoned the school authorities, bus driver Doan Quy Phien, and Nguyen Bich Quy, an assistant traveling with the students on the bus for questioning.

"Based on initial testimonies, we've decided that there are signs of involuntary manslaughter. The responsibility will for sure involve Quy," Lieutenant Colonel Tran Van Hoa, Deputy Chief of Cau Giay District police said at the conference.

Long was put on the school bus at 7 a.m. on Tuesday. Teachers from the school said that Long sat on the last row of the 16-seat car, which was carrying 13 students on Tuesday morning.

When the bus arrived at school, two other students who were on their first day to school were crying a lot, and Quy was busy helping them and did not check the number of students stepping down.

Quy and Phien failed to check the vehicle until they picked up the students from school in the afternoon.

Long was then found lying on the car's floor, behind the driver's seat.

Hanoi's temperatures on Tuesday were between 26 and 34 degrees Celsius (79-93 degrees Fahrenheit).

If a person is found guilty of involuntary manslaughter, she/he could face up to 10 or 12 years in prison.

The Gateway International school, founded in 2015, offers classes from first to 9th grade. It has three campuses, two in Hanoi and one in the northern port city of Hai Phong.

It charges more than $5,000 in tuition fees a year for primary school students apart from charges for meals, school bus service and after school sitting services, according to the school's website.

Vietnam's GDP per capita last year was $2,560.