The suspect arrested over Tuesday’s parcel bomb attack at Yonsei University has admitted to plotting the blast, taking a cue from a subway terror attack in St. Petersburg, police said Wednesday. He said he made the explosive by himself.

Seodaemun Police Station arrested the 25-year-old engineering graduate student near the university campus in Seoul late on Tuesday night, about 12 hours after the bomb went off, causing minor burns to the professor who opened it.

Investigators said the suspect, surnamed Kim, confessed to making the explosive, inspired by a bomb filled with shrapnel used in the explosion in Russia in April.

Further investigation is required on his motives as well as whether he intended to kill the professor, police said.

Regarding where he got the information on how to make the bomb, Kim reportedly said that he manufactured the bomb in late May on his own in his room, without watching online tutorial videos. Police will review records of Kim’s usage of his smartphone and laptop to check if this is true.

The explosive Kim allegedly made was a nail bomb, according to officers. It was made from a coffee tumbler, gunpowder and four AA-size batteries. It also contained scores of screws 1 centimeter long or shorter to make it more lethal.

Police said they will request a warrant to formally arrest the suspect.

Meanwhile, local media reported that the suspect may have acted out of a personal grudge against the 46-year-old professor, citing classmates.

He had complained of a heavy workload studying under the professor and seemed to be under stress to achieve higher English test scores in order to apply for a substitute program for his mandatory military service.

“The professor had entrusted various school work to Kim, but Kim frequently complained about having too little time to study English,” a local media outlet quoted one of Kim’s classmates as saying.

But the police said Kim did not say that he was on bad terms with the professor.

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