A 43-year-old man was arrested Thursday on suspicion of posting an online message threatening Princess Kako, the 20-year-old granddaughter of Emperor Akihito, police said.

Toshiyuki Ikehara, an unemployed man from Tokyo, allegedly posted the message on the 2channel online bulletin board on May 16, prompting the officers protecting the Imperial family to tighten security.

Ikehara turned himself in to the police on Wednesday night after learning about reports of his alleged post, according to the police.

Princess Kako is the second daughter of Prince Akishino, and is enrolled at the International Christian University outside central Tokyo. Partly as a result of her increased exposure to the public, details of her activities have been widely reported in tabloids and other media.