Prosecutors investigating election fraud involving the support base for Takeshi Tokuda have questioned the House of Representatives member on a voluntary basis, investigative sources said Tuesday.

Monday's questioning apparently focused on whether the 42-year-old Liberal Democratic Party lawmaker knew that the Tokushukai hospital and welfare chain, founded by his father, covered the costs of group employees who worked on his campaign staff during the general election last December. The election law bans such payments.

More than 200 Tokushukai group employees were sent to Kagoshima, the biggest city in Tokuda's Kagoshima No. 2 constituency, after the Lower House was dissolved Nov. 16, and visited voters' homes and made telephone calls asking the electorate to vote, Tokushukai sources said.

On a visit to his local office, Tokuda urged employees to be careful and to "behave in a mature way that would lead to votes," the sources said.

The sources said that group employees have been involved in election campaigning dating back to the time Tokuda's father, Torao, 75, was a Lower House member, and that it also occurred when the incumbent, who served as his father's state-paid secretary between 2002 and 2005, was first elected in 2005 and re-elected in 2009, they said.

Investigators at the Tokyo District Public Prosecutor's Office raided Tokuda's office and home in Kagoshima on Sept. 20 and his parliamentary office in Tokyo on Friday.

They have also questioned Tokuda's father, mother and two sisters, all of whom denied knowledge of the hospital chain making up for the employees' work absence, as well as senior officials of group hospitals.

Tokuda has refrained from responding to the allegations.