Two men suspected of being behind a string of robberies across Japan were taken into police custody en route to Tokyo on Thursday following their deportation from the Philippines.

The return of Yuki Watanabe, 38, and Tomonobu Kojima, 45, means all four suspects who were detained in the Southeast Asian country on local charges are back in Japan, enabling police to begin full investigations into the robberies that have shocked the country.

A car transporting one of the four robbery suspects deported from the Philippines arrives at a police station in Tokyo's Shibuya Ward on Feb. 9, 2023. (Kyodo)

The four are also suspected to have been leading members of another crime group that operated a scam targeting elderly people in Japan. The police are continuing to build their case regarding the robberies and have initially arrested the four on allegations of stealing cash cards in 2019 in relation to the fraud ring.

Watanabe is believed to have led the fraud group from the Philippines, while Kojima is thought to have managed its revenue collection. The group is estimated to have stolen over 6 billion yen ($45.6 million) before appearing to switch its activities to robberies, according to the police.

The other two members of the group, Kiyoto Imamura and Toshiya Fujita, both 38, were deported from the Philippines to Japan and arrested on Tuesday.

The four are believed to have remotely coordinated the robberies from the Philippines by buying privileges at the immigration facility where they were detained. They are thought to have organized the crimes via an encrypted messaging app through which they offered high-paid "part-time jobs" to people tasked with committing the robberies.

Based on testimony from suspects under arrest and similarities in methods, the robbery group is believed to be linked to more than 50 incidents in 14 prefectures starting in summer 2021.

Recent robberies include an incident resulting in the murder of a 90-year-old woman in Tokyo in January, with investigative sources saying Thursday three persons of interest have been identified in the case.

All three appear to have conducted the attack and are currently under arrest in connection with other incidents across the country, the sources said.

Examinations of footprints at the scene suggest no fewer than four people were involved in the incident that led to Kinuyo Oshio's death. The police believe that the items stolen from her include a ring and a luxury watch.

The four suspects returned to Japan likely include the person, or persons thought to have used the pseudonyms "Luffy" and "Kim" when the crimes were allegedly committed.

The police plan to get to the bottom of the robberies by analyzing around 15 smartphones and tablets seized from the four at the immigration facility in the Philippines.