Police in Japan have opened an investigation after finding two knives near the school bench of Emperor Akihito's 1
The knives were discovered Friday in a classroom of a high school attended by Prince Hisahito.
Police inspect CCTV footage of a man entering the school grounds.
Prince Hisahito will be second in the series following Emperor Akihito's abdication next week.
The police believe the unidentified man was caught in front of the camera, dressed in blue and wearing a helmet, posing as a construction worker to gain access to the Ochanomizu University building.
Prince Hisahito and his classmates were in another part of the school when the knives were supposedly planted.
While the prince is often guarded by police, they do not escort him to school classes, an Imperial Household Agency official told Kyodo News.
Investigation sources told Kyodo that there was a piece of tape on each desk in the classroom displaying the name of the student, making the Prince's seat recognizable.
They said that the blades of the knives were painted pink.
The incident comes as Japan prepares for the abdication of Emperor Akihito on Tuesday and ends his 30-year reign.
The 85-year-old will voluntarily resign from his first abdication since 1817 for health reasons.
His son, Crown Prince Naruhito, will climb the Chrysanthemum throne the following day.
Emperor in Japan has no political power, but serves as a national symbol. The imperial family is universally popular and threats against them are relatively rare.