A supporter of Fukushima Gov. Eisaku Sato allegedly played a crucial role in rigging bids for prefectural projects, sources said Saturday.

Prosecutors are expected to launch a full-scale investigation this week, and some Fukushima construction company officials have already admitted they arranged the winners of public works projects, the sources said.

The 59-year-old supporter of Sato is said to have been deeply involved in some prearrangements that decided which of the companies would win orders even before the prefectural government invited bids for the projects, according to the sources.

One of the sources cited involvement by a former senior prefectural official in the alleged bid-rigging.

"Such arrangements may be regarded as bid rigging," a senior official of a construction firm said after being questioned by prosecutors on a voluntary basis.

Another figure in the construction industry said prefectural officials "tacitly tolerated" such practices by contractors.

A special squad of the Tokyo District Public Prosecutor's Office is investigating the process of determining project winners in Fukushima in connection with suspected tax evasion by Mizutani Kensetsu Co., an engineering firm that worked on the projects as a subcontractor.

The projects involved construction of a road linking the Tohoku Expressway and the Banetsu Expressway and a sewage system in the lower region of the Abukuma River.

Two former executives of Mizutani Kensetsu have been indicted for corporate tax evasion.

The prosecutors have already questioned a former secretary to a retired Diet member, senior officials of Maeda Corp. and other general contractors on suspicion that a slush fund created through the tax evasion was used to lobby politicians and pay off organized crime groups.

A 58-year-old branch office head of Tokyu Construction Co. committed suicide last month after being questioned by prosecutors in connection with the alleged tax evasion.