Sources say Tokyo prosecutors have questioned Hagiuda Koichi, a former policy affairs chief of the main ruling Liberal Democratic Party, on a voluntary basis.

The move is part of an investigation into a political fundraising scandal involving the party's largest faction, which was once led by the late Prime Minister Abe Shinzo.

The Abe faction is suspected of having paid kickbacks to the offices of member lawmakers who sold tickets for fundraising events in excess of quotas without recording the excess revenue in its political funds reports.

Prosecutors last week raided the offices of the Abe faction and the Nikai faction, which is similarly suspected of violating the political funds control law.

The offices of most Abe faction lawmakers -- including Hagiuda and five other executive members -- are said to have received the kickbacks.

Hagiuda was recently replaced as chair of the LDP's Policy Research Council.

Four other faction executives have also undergone voluntary questioning, including former Chief Cabinet Secretary Matsuno Hirokazu, former LDP Diet Affairs Committee Chairperson Takagi Tsuyoshi, former LDP Upper House Secretary-General Seko Hiroshige and former education minister Shionoya Ryu.

It's believed they were questioned about their views as faction executives and as lawmakers whose offices received the kickbacks.