TOKYO (Kyodo) -- A Japanese court on Thursday sentenced a man to life in prison for his key role in a robbery resulting in the death of a woman in western Tokyo, suspected to be part of a series of burglaries across the country orchestrated by a man using the pseudonym "Luffy."

In handing down the ruling at the Tachikawa branch of the Tokyo District Court, Presiding Judge Akira Sugawara said Rikuto Nagata "acted as the leader of those carrying out" last year's robbery that led to the death of a 90-year-old woman.

"He gave orders to other perpetrators based on the situation at the robbery scene," Sugawara said, adding the role of Nagata, 23, was also "considerably large in the series of robberies."

At least 50 robberies and thefts suspected to be connected to a group have occurred across over a dozen prefectures since 2021. This case, however, was the only robbery that resulted in a death, striking a nerve among the public in Japan where such violent crime is rare.

The sentence was in line with the prosecutors' demand. While Nagata's defense team sought a fixed prison term, Nagata himself asked to be executed.

During trial, he had admitted to the charges including the robbery resulting in death.

According to the ruling, Nagata and Hiroyuki Nomura, 53, conspired with others to enter the woman's home by pretending to be parcel deliverymen.

They kicked her and beat her with a crowbar, causing her to die, while stealing items such watches, the ruling said.

Nagata was also involved in five other robberies, including in Tokyo's Nakano Ward and the city of Hiroshima, according to the ruling.

He "ordered his accomplice to beat the woman with a crowbar" in what can be regarded as "torture," the presiding judge said, describing the action as "relentless and ruthless."

Sugawara added that despite Nagata's apology, he should not be given a fixed prison term due to the gravity of the crime.

While those who gave the instructions called themselves Luffy, "Kim," and "Mitsuhashi," among other pseudonyms, perpetrators of the actual robberies were individuals enticed via social media with promises of lucrative "yami baito," literally meaning "dark part-time work."

Police suspect one of the four ringleaders, Kiyoto Imamura, 40, who was indicted late last year, used the pseudonym Luffy when planning the crime and instructing subordinates.