The Department of Special Investigation (DSI) will widen its investigation into the collapse of the State Audit Office (SAO) building to include companies responsible for the design, building supervision, and construction contracts.
The department will also probe the forged signature claims of a senior engineer linked to the project.
Justice Minister Pol Col Tawee Sodsong and DSI director-general Pol Maj Yutthana Praedam on Wednesday held a meeting at the site of the collapse to follow up on the investigation against the two project contractors.
The investigation has been split into two cases. One deals with alleged business nominees and the other involves the forged signatures of Somkiat Chusangsuk, a senior engineer who claimed no involvement in the project.
The DSI has accepted both as special cases.
The DSI is investigating the two contractors, suspected of using nominees to win contracts for the project.
The investigation needs to scrutinise evidence including four contracts regarding the building's structural design, construction supervision and design modifications.
As for the signature forgery case, the DSI says it has obtained the information and evidence required. It has sent the allegedly forged signatures and authentic ones to forensic experts for comparison, with the results expected within two weeks.
Mr Somkiat said he was falsely named as the "project supervisor" for PKW Joint Venture in documents related to design modifications to the building's elevator shaft, despite having stopped working as a project supervisor more than 20 years ago.
He is pursuing legal action against those involved in the alleged forgery and has supplied evidence to both the DSI and police in a bid to clear his name.
The DSI is required to complete its probe into the nominee case before it can expand its investigation to cover other cases, such as the bidding for the project's contracts.
Meanwhile, Worasak Kanok-Nukulchai, an engineering expert and Asian Institute of Technology emeritus professor, described the tragedy as a "pancake collapse" which took eight seconds.
The floors collapsed sequentially, one on top of the other, like a stack of pancakes.
The collapse has claimed 44 lives so far, with 50 more people still missing.
