Ruling party lawmaker Mito Kakizawa is under investigation on suspicion of vote-buying to elect a mayoral candidate in a Tokyo ward in April, sources close to the matter said Friday.

Several secretaries of Kakizawa, a House of Representatives lawmaker of the Liberal Democratic Party, offered and distributed cash to several Koto Ward assembly members, the sources said.

Although Kakizawa has said there was no illegality as the money was provided to encourage them ahead of the ward assembly election, also held in April, prosecutors suspect he intended to buy votes in violation of the election law, the sources said.

Prosecutors searched the constituency office of the 52-year-old lawmaker and other locations Thursday, the sources said.

Several ward assembly members have already been questioned about the payments on a voluntary basis, according to the sources.

Kakizawa, the son of a former foreign minister, resigned as senior vice justice minister late last month after it was reported he had proposed to Yayoi Kimura that she use an online advertisement during her successful mayoral campaign in the ward, an alleged violation of the election law.

Kimura previously served as a lower house lawmaker of the LDP, currently headed by Prime Minister Fumio Kishida.

On Wednesday, Kimura stepped down as mayor for using a paid advertisement on YouTube to urge people to vote for her.

Japan's public offices election law prohibits the use of paid advertisements online for specific candidates, rather than political parties.

Kimura said she decided to use the online advertisement after Kakizawa suggested it, arguing she did not know it was illegal, the sources said. She has denied involvement in the distribution of the money, they said.