Tokyo (CNN)Police in Japan have tightened security around Emperor Akihito's 12-year-old grandson after two knives were found near his school desk.
The knives were discovered on Friday in a classroom at the prestigious Ochanomizu University Junior High School, attended by Prince Hisahito.
The mysterious discovery comes just days before Hisahito's uncle, Crown Prince Naruhito, is due to ascend the throne, following the abdication of Emperor Akihito this Tuesday.
According to reports, Hisahito, who is third in line to the throne, but will become second in line after his uncle's ascension, was in a different part of the school building when the knives were discovered.
The two kitchen knives, which had been taped to either end of a stick and whose blades were reportedly painted pink, were discovered balanced between Hisahito's desk and his neighbor's, Japan's Asahi Shimbun reported. Nothing indicating a motive, or a claim of responsibility was left with the contraption.
Each desk has the name of its occupant written on it, making the prince's desk easily identifiable, the newspaper said.
Security footage from the school appeared to show a middle-aged man, wearing the uniform and helmet of a construction worker, on school premises around the time that the knives were discovered.
The school addressed the incident with a statement on its website, pledging to review its security protocols.
"We deeply apologize to have caused the great concern to everyone with regards to the incident at Ochanomizu University Junior High School," the statement, attributed to the university's president, Kimiko Murofushi, says.
"Ochanomizu University will review security measures urgently, in coordination with the junior high school, and work to secure its safety so that this kind of incident will never happen again."
While Hisahito has a police detail, they do not accompany him into class, according to reports.
The 12-year-old prince began studying at the school last month, Japan's national broadcaster NHK reported, after graduating from the university's affiliated elementary school.
Emperor Akihito will step down from the Chrysanthemum Throne -- the first abdication from the Japanese throne in 200 years -- on Tuesday.
Hisahito's uncle, Crown Prince Naruhito, will become the country's 126th emperor when he is crowned in a ceremony on Wednesday.
The 12-year-old is the only male grandchild of Emperor Akihito and Empress Michiko. Given Japan's male-only hereditary laws, he will become second in line to the throne after his father becomes the crown prince and successor this week, when Naruhito ascends.
In August 2016, Akihito gave a rare televised address, where he said his age and fitness level could make it "difficult" to carry out his duties in the future, a plea many took as a request to step aside.
Following that speech, the Japanese parliament in June passed into law a historic bill to allow 83-year-old Akihito to abdicate. Japan is the oldest hereditary monarchy in the world, dating back 14 centuries.
Akihito himself is a direct descendant of Japan's first Emperor, Jimmu, believed to have reigned around 660 BC.