Four men were arrested Wednesday on suspicion of carrying out a fatal robbery at a 90-year-old woman's home in western Tokyo in January, a case believed to be linked to a string of burglaries seen across the country over the past year or so.

The series of break-ins are likely to have been ordered by a person or persons thought to have used the pseudonyms "Luffy" and "Kim" and to be among men recently deported to Japan from the Philippines.

Hiroyuki Nomura, a 52-year-old resident of Saitama Prefecture, was arrested on suspicion of the murder and robbery of Kinuyo Oshio in the city of Komae on Jan. 19, according to investigative sources. Shogo Fukushima, a 34-year-old also from Saitama, was arrested on suspicion of acting as an accessory to the crime.

Investigators examine on Jan. 23, 2023, the area around the residence where Kinuyo Oshio was found dead following a robbery in Komae, Tokyo. (Kyodo)

The two others arrested the same day are both suspected perpetrators -- Rikuto Nagata, a 21-year-old resident of Kanazawa, Ishikawa Prefecture, and a 19-year-old male university student who resides in Tokyo's Nakano Ward.

Nagata was indicted in December last year for a robbery case in Nakano, while the student, whose name is withheld by law pending a formal indictment because he is a minor, was arrested in Ishikawa on suspicion of using a fake driver's license.

Nomura and the others are suspected of breaking into Oshio's home before beating her to death and stealing three luxury wristwatches and a diamond ring, while Fukushima is suspected of acting as an accomplice by renting two cars the day before.

The police later found the cars, one of which had a smartphone left inside. The messages on the phone included what appear to be instructions sent from "Kim" using the encrypted messaging app Telegram.

The police suspect that "Kim" and "Luffy" could be the aliases of one or more of a group involving Yuki Watanabe, 38, and three others who were deported from the Philippines earlier this month. They have so far been arrested on suspicion of theft in connection with scam cases.

The four deportees are believed to be linked to at least 20 robberies that have occurred across more than a dozen prefectures. The murder and robbery of Oshio, in particular, struck a nerve among the Japanese public due to such criminal violence being rare in Japan.

Police are examining smartphones and computer tablets confiscated from the men while they were held at an immigration detention facility in Manila. According to investigative sources, however, some of the devices have been reset, with the data on them wiped.

"Even if messages are found on the devices or we are able to recover them somehow, there is a possibility that those smartphones were shared among them," a senior investigator said. "We'll likely face a high hurdle identifying who sent which messages."

Given that the four were potentially able to coordinate their stories before their deportation, the investigator said police are prepared for what is likely to be a "long and hard" investigation.

Related coverage:

2 more robbery suspects arrested, land in Tokyo from Philippines

Robbers of east Japan homes suspected of striking other areas