Photo/Illutration Hirokazu Matsuno announces his resignation as chief Cabinet secretary position on Dec. 14. (Takeshi Iwashita)

Tokyo prosecutors have requested voluntary interviews with Hirokazu Matsuno and Hiroshige Seko, two ruling party heavyweights who recently resigned from their high-level posts over a money scandal, sources said.

Investigators are expected to question the two on a voluntary basis along with other senior members of the Abe faction in the Liberal Democratic Party by the end of the week, the sources said.

The Tokyo District Public Prosecutors Office is investigating the Abe faction over its suspected failure to report about 500 million yen ($3.5 million) in revenue gained through sales of tickets to its fund-raising parties over the past five years.

Investigators suspect the scheme was systematic, and are trying to determine who was behind it and why.

Matsuno, who stepped down as chief Cabinet secretary over the scandal on Dec. 14, had served as secretary-general of the faction and was in a position to oversee its financial matters.

He and Seko, who resigned as secretary-general of the LDP’s Upper House caucus, are each believed to have received more than 10 million yen of the funds from the faction and failed to record the money in their fund reports, the sources said.