A 36-year-old man was arrested Sunday over a long-running attempt to pressure bookstores and major convenience store chains to stop selling a popular manga series and related goods.

Hirofumi Watanabe, an Osaka resident, has admitted sending more than 400 threatening letters over a year to the retailers and news media in his campaign against the comic series “Kuroko no Basuke” (“The Basketball which Kuroko Plays”) by Tadatoshi Fujimaki, the Tokyo Metropolitan Police said.

Watanabe was apprehended as he was about to mail additional letters near Yebisu Garden Place, a popular shopping area in Shibuya Ward, after arriving in the capital earlier in the day.

“I lost the game,” he was quoted as saying during his arrest.

According to police, Watanabe was filmed by security cameras near several locations related to his threats.

Among the letters was one sent to Kyodo News saying that a poisoned food item had been placed in a convenience store.

He was arrested on suspicion of placing an extortion letter and a container of hydrogen sulfide, a colorless gas that smells like rotten eggs, in a gymnasium at Sophia University in Tokyo, where Fujimaki once studied, on the evening of Oct. 12 last year, disrupting the school’s operations, the police said.

He is believed to have started sending the threatening letters the same month. He told investigators that while he was not personally acquainted with Fujimaki, he was jealous of his manga success, the police said.

The intimidation led some bookstores and convenience stores to pull the comic books and related DVDs from their shelves.

The series, which focuses on high school basketball player Tetsuya Kuroko and his team, has been published in serial form in a popular weekly magazine since 2008. Total sales of book versions have topped 23 million copies.