The Monday arrest of a man in connection with the murder of a 7-year-old girl in central Japan last week has shocked people who knew him as an "earnest worker" and a "mature" guy.

Haruka Kobayashi, 23, who was arrested on suspicion of abandoning and damaging the body of Tamaki Omomo in the city of Niigata, has hinted at his involvement in the killing of the second-grade elementary school student, according to investigative sources. His case was sent to prosecutors on Tuesday.

The president of an electrical engineering company that employs Kobayashi in the city said the suspect was "an earnest and quiet employee" who had never missed work without prior notice until May 7, the day the girl's body was struck by a train.

An autopsy has found she had been strangled to death sometime before the impact.

(Haruka Kobayashi)

Kobayashi, who lives in the same neighborhood as the victim, failed to show up to work that day and sent an email to the company the next day saying he was feeling unwell. He never returned to his place of employment, according to the company president.

The suspect joined the firm after graduating from high school and was in his sixth year there, according to the president.

The company president said he reacted with disbelief when he first heard of Kobayashi's arrest.

Kobayashi has already admitted to abandoning the girl's body on the track of the JR Echigo Line in Niigata's Nishi Ward between 10:20 p.m. and 10:30 p.m. on May 7 before the body was struck by the train. He is believed to have been unacquainted with the girl, according to the sources.

The homes of Kobayashi and the girl are separated by only 100 meters and the site where the victim's body was found is just 70 meters from where the accused lives with his family. Local residents have laid flowers and sweets in tribute to the girl near the site.

Kobayashi went to the same elementary school as the victim and belonged to a science club when he attended junior high school, according to people who knew him.

"He was good at building robots," recalled a 24-year-old man who attended junior high with Kobayashi. "I remember him smiling after being recognized for (robot building) in a school assembly."

The man described Kobayashi's personality as "mature" and "not gloomy."

"He had friends and was not left out at school. I didn't have the impression that he was a strange child," said a woman who also knew him when he was a junior high school student.

The arrest brought relief to worried parents of students attending the same elementary school and many voiced their sympathy for the parents of the victim.

"I am relieved. I hope peace will gradually return to the neighborhood," said a 43-year-old father who accompanied his daughter to the school where she is in the sixth grade.

"When I think about how the (girl's) parents are feeling, I cannot rejoice over the suspect's arrest," said a woman in her 20s whose child is in first grade.

Yutaka Hasegawa, the principal of the girl's elementary school, said in a statement he remains "outraged" over the incident and vowed to "make an all-out effort to get the children's everyday lives back to normal as soon as possible."