Opposition leader Ichiro Ozawa's aides, including arrested secretary Takanori Okubo, had construction companies get employees to work for Ozawa's election campaigns, contractor sources said Thursday.
Prosecutors are questioning construction officials in the Tohoku region, Ozawa's stronghold, on suspicion that such labor was a token of the close ties between the local construction industry and the president of the Democratic Party of Japan, the biggest opposition party.
Ozawa is said to wield strong clout over public works projects in the area, investigative sources said.