The Department of Special Investigation (DSI) has summoned representatives from 65 companies for questioning after learning they may have been involved in bid-rigging with the construction business owned by Praween Chanklai, aka "Kamnan Nok".
Mr Praween was arrested in Muang district of Nakhon Pathom after Pol Maj Sivakorn Saibua, an officer of Highway Police Division 2, was slain by a gunman during a dinner party at his house in tambon Takong in Muang district of Nakhon Pathom last Wednesday.
The gunman, who was Mr Praween's close aide, was identified as Thananchai or "Nong Tha Pha" Manmak. Thananchai died in an extra-judicial killing.
However, Mr Praween denied he had ordered his subordinate to shoot and kill Pol Maj Sivakorn in a moment of anger for rebuffing a transfer request for his nephew, who is a policeman.
Mr Praween inherited his construction business from his father, a former headman of Tambon Takong who established the P Raweekanok Construction Company. It won several road, building and bridge construction projects, as well as providing canal dredging and rental services in Nakhon Pathom and several neighbouring provinces.
Mr Praween later set up another company, the P Phatanarungrod Construction Company.
Pol Capt Surawut Rangsai, director of the DSI's Division for Offences Relating to the Submission of Bids to Government Agencies, said on Tuesday that two companies that Mr Praween owns -- P Raweekanok Construction Company and P Phatanarungrod Construction Company -- may have been involved with violating the regulations on a bid submission for government agency projects.
The investigation by financial analysts also showed the account balances of Mr Praween's companies were abnormal when compared to the annual statements submitted to the Department of Business Development, he said.
Since 2015, both companies have won bids on 1,300 government projects valued at over 70 billion baht, which contrasts with the companies' assets, which are said to be worth 100–200 million baht, Pol Capt Surawut said.
The division has also issued warrants to 65 companies that had bids on highway projects in 2017 for questioning next Monday after bidding issues were found, he said.
A potential financial transaction between Mr Praween and the gunman was uncovered after at least six police at the party were found to have made or received unusually large transfers, suspected of being payment for the shooting, that evening.
Meanwhile, the death of Pol Col Vachira Yaothaisong, 44, Sivakorn's commander, was reported as a suicide after traces of gunshot residue were found on both of his hands, said national police chief Pol Gen Damrongsak Kittiprapas.