Prosecutors are considering asking Prime Minister Yukio Hatoyama's mother, 87, to submit a written statement on her alleged contribution of a large amount of money to him, sources said.
Hatoyama's mother allegedly provided him with more than ¥1 billion over six years up to 2008, some of which is believed to have been recorded as political donations from fictitious donors.
Kunio Hatoyama, the prime minister's younger brother and a former internal affairs and communications minister, also received political funds from their mother.
The Tokyo District Public Prosecutor's Office is expected to ask the prime minister to submit a written statement so it can establish a criminal case over the alleged falsification of his funding reports.
It is unlikely that Hatoyama himself will be indicted, but prosecutors are aiming to file an indictment against his 59-year-old former secretary without arresting him, according to the sources.
Kunio Hatoyama earlier expressed his intention to pay gift tax on the money. The prime minister also showed his willingness to pay the gift tax.