The suspect detained over Tuesday’s parcel bomb attack at Yonsei University confessed that he acted out of a personal grudge against his supervising professor, police said Thursday.

“Kim said he felt offended when he had arguments with the professor over his thesis and he planned the crime in late May after he was harshly rebuked by the teacher,” Seodaemun police said during a press briefing. The professor had been the suspect’s supervisor since the beginning of his postgrad course.

Kim, a 25-year-old engineering graduate student, also confessed to making the bomb by himself, inspired by a subway terror attack in St. Petersburg, Russia, in April.

A parcel containing a nail bomb was sent to the professor’s office early Tuesday morning. It went off when the professor tried to open the parcel, causing minor burns.

The explosive was made from a coffee tumbler, gunpowder and four AA-size batteries, and contained scores of screws 1 centimeter long or shorter in it, police said.

The suspect, nabbed late Tuesday night, was formally arrested Thursday on charges of illegal use of explosives, not attempted murder. Police said further investigation is needed to establish whether he intended to kill and whether the “clumsily-made” bomb should be seen as a murder weapon.

The injured professor said he does not want the student to face punishment.

By Kim Da-sol (ddd@heraldcorp.com)