Shares in Petrofac (POFCF) tanked Thursday, falling almost 30%, after the oil services group said it had suspended its chief operating officer until further notice amid a probe into potential bribery and corruption at the group.

London-listed Petrofac gave no details of the reasons for its decision to relieve COO Marwan Chedid of his responsibilities, but noted that the decision had resulted in his resignation from the board.

The company also said that it had formed a special committee of the board to oversee its response to the investigation and will appoint of an external expert to manage its interactions with the U.K.'s Serious Fraud Squad (SFO).

Petrofac shares finished the day down 29.95% at 430.80 on their Wednesday close, making them the worst performer on the UK All-share Index.

"These decisions signal the Board's determination to cooperate fully with the SFO and its investigation, whilst ensuring Petrofac continues to deliver for its clients," said Chairman Rijnhard van Tets.

The SFO, on May 12, announced an inquiry into Petrofac and its subsidiaries, citing suspicions of "bribery, corruption and money laundering" linked to a sprawling probe into the activities of Monaco-based oil company Unaoil, a provider of oil field services in the Middle East, Central Asia and Africa.

The investigation of Unaoil has also ensnared the UK subsidiaries of Swiss engineering group ABB (ABB) , and Houston-based engineering and construction firm KBR (KBR) , both of which are under investigation by the SFO as a result of their interactions with Unaoil.

Petrofac last year conducted its own investigation into its dealings with Unaoil and said it had found no evidence of bribery payment. The company worked with Unaoil from 2002 until 2009, primarily in Kazakhstan where the Monaco-based business provided local consultancy services.

Earlier this month, Petrofac's CEO Ayman Asfari and its COO were arrested, questioned and released without charge by the SFO as part of the investigation. Asfari will continue in his role as CEO but will play no role in the investigation by the SFO, Petrofac said on Thursday.

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