Prosecutors raided the main offices of Samsung BioLogics and its accounting firms on Thursday as part of a probe into alleged accounting fraud.

Investigators from the Seoul Central District Prosecutors' Office searched the accounting division of the biotech company in Incheon, west of Seoul, to confiscate evidence.

Separate raids were carried out at Samsung Bioepis, Samsung C&T and four accounting firms, including KPMG Samjong Accounting and Deloitte Anjin LLC.

The raids come about a month after the Financial Services Commission ruled that Samsung BioLogics intentionally violated accounting rules ahead of its initial public offering in 2016. The case has been referred to the prosecution.

The alleged irregularities reportedly helped boost the value of Cheil Industries and benefited Samsung Group's heir apparent, Lee Jae-yong, as Cheil Industries held a 46-point-three percent stake in Samsung BioLogics and Lee was a major stakeholder in Cheil at the time. 

Market watchers and activists suspect the alleged accounting irregularities are related to the 2015 merger of Cheil and Samsung C&T, which was widely seen as a step to enhance Lee's control of Samsung Group.