Representatives from 20 firms which took part in bidding for state projects ultimately won by companies owned by Praween Chanklai, also known as Kamnak Nok, were questioned by the Department of Special Investigation (DSI) on Monday.
The DSI is looking into the bidding for the construction of a section of Highway 375, which links Don Toom and Lam Luk Bua, back in 2017, as well as the section of Highway 375 which links Lam Luk Bua with Highway 346 in 2021.
An initial investigation suggests that a construction company owned by Mr Praween colluded with others to manipulate the bidding for the two projects.
Mr Praween's construction business became the focus of an investigation after he became a key suspect in the fatal shooting of highway policeman Pol Maj Sivakorn Saibua at a dinner party at his house in Nakhon Pathom.
His firm won several road, building and bridge construction projects, and provided canal dredging services in Nakhon Pathom and several neighbouring provinces.
According to the DSI, 33 firms bought bid envelopes for the Don Toom-Lam Luk Bua project, but only four took part in the bidding. Thirty-two firms bought bid envelopes for the other project, but only three firms joined the bidding contest.
Pol Capt Surawut Rangsai, director of the DSI's division on collusion in state projects, said that these firms were summonsed for questioning as witnesses. The rest are scheduled to meet the DSI today and tomorrow.
Interior Minister Anutin Charnvirakul on Monday vowed to personally look into the alleged irregularities in projects carried out by the Interior Ministry. He said he would pay special attention to projects which went ahead despite numerous complaints.
National police chief, Pol Gen Damrongsak Kittiprapas, said on Monday that strict screening would be in place after Mr Praween was revealed to be a member of Nakhon Pathom's provincial committee on monitoring police work.
Nakhon Pathom governor, Surasak Charoensirichote, said he had removed Mr Praween from the committee and that a probe was underway to figure out how he became a member.