An online threat claiming a bomb had been sent to Yonsei University in Seoul sparked a midnight scramble for police and firefighters, resulting in the evacuation of some 100 people on campus.

Police received a report at 11:38 p.m. Wednesday of an online post that claimed a package carrying a bomb would be delivered to the campus, according to Seodaemun Police Station and Seodaemun Fire Department on Thursday.

The post had been uploaded at around 10 p.m. on the bulletin board for Yonsei University on Everytime, an online community widely used by Korean university students, with the title reading: “Pissed off about a professor.” The writer said, “I have put a bomb in a tumbler and sent it by delivery.” The post has since been removed.

About 20 police officers, including a commando bomb disposal unit, and 33 firefighters were deployed to search four of the university’s engineering buildings for two hours. All packages sent to the campus were examined, but no explosives were found, police said.

Meanwhile, Yonsei University experienced a similar incident in June 2017, when a graduate student injured a professor with an improvised explosive device that he had made.

The graduate student reportedly planned the crime after taking insult by the professor for feedback on his thesis paper. The student placed a bomb in a paper bag that he hung on the door knob of the professor's office. The bomb exploded as the professor opened the door, resulting in first- and second-degree burns to his arm.

The offender was sentenced to two years in prison in April 2018.