The offshore arm of Australian company Leighton Holdings paid millions of dollars in bribes to middlemen as part of an audacious strategy to influence Iraq's deputy prime minister, oil minister and other senior officials, and win more than $1.3 billion of oilfield contracts.
Leighton's 18-month campaign of corruption, bribery, fraud and money laundering is revealed in the biggest leak of documents in the oil industry's history. 
The documents are leaked from the email account of Cyrus Ahsani, the chief executive of the global oil industry's bagman, Unaoil. Unaoil is a Monaco-based company that specialises in paying bribes. It was paid tens of millions of dollars by Leighton in 2010 and 2011 to help the Australian company win Iraqi government contracts.
The documents reveal that Russell Waugh, former managing director of Leighton Offshore, a division of Leighton Holdings, was directly involved in corruption, and that former Leighton Holdings chief operating officer David Savage and senior executive Peter Cox backed corrupt activities.
The evidence will revive a long-running Australian Federal Police bribery investigation into the company and its former executives, which began in 2011 but is yet to lead to any charges.
Fairfax Media can reveal that Leighton Offshore:
" through Unaoil or other middlemen, bribed senior Iraqi officials, with the implicit or explicit knowledge of Mr Waugh or Mr Cox, to win contracts and extensions to an existing contract in 2010;
" set up their own relationships with corrupt officials in Iraq's government-run South Oil Company;
" backdated an invoice and used a money-laundering hub in the UAE to transfer a $5.6 million bribe and paid an $800,000 fee to Unaoil to facilitate the transaction.
Hundreds of emails from Unaoil show that high ranking Iraqi oil officials Dhia Jaffar al-Mousawi and Oday al-Quraishi were put on secret retainers or paid large bribes in return for supporting Leighton Offshore.
In 2010, Ahmed al-Jibouri, a Jordanian middleman code-named the "Doctor" was promised up to $20.5 million in bribes by Unaoil to influence top Iraqi officials, including oil minister Abdul-Kareem Luaibi and then deputy prime minister Hussain al-Shahristani, to support Leighton Offshore.
The Unaoil emails state that the "Doctor" and his contacts had the power to decide whether or not Leighton or its competitors won major contracts.
"We must not underestimate that doctor and his group ability [sic] to have the final say ... No matter how unpleasant we find it. Once they have made up their minds, it seem difficult to budge them," a Unaoil internal email states.
Another email reports that the Doctor was "going to Baghdad tonight" to talk to his political contacts about whether a $6 million bribe would convince them to deliver another contract to Leighton. "I will talk to group again to bring CM [a contract sought by Leighton] back on the table. I made it 6 [million dollars]."
Dr Shahristani has denied any wrongdoing, said he did not know Jibouri, and had only heard of Unaoil from media reports.
However, the Unaoil emails show that the bribe money came out of fees of more than $75 million that Leighton promised to pay. Part of this was a "marketing fee" for helping it win the pipeline contracts, and part was for Unaoil's role as a subcontractor on the contract.
During negotiations over Unaoil's fee for a second pipeline contract, an internal Leighton file note from   November 2010 acknowledged that these fees included kickbacks, which were "too much money and a clear lack of value for money".
Direct evidence of corruption involving former COO David Savage emerges in the payment of a $5.6 million fee, which was paid by Unaoil to corrupt Iraqi officials. The fee helped secure Leighton a $66 million contract extension to its project without the need to go to a second competitive tender.
Unaoil emails reveal that Savage and Waugh were involved in laundering this $5.6 million through a UAE company called Asian Global Projects and Trading. As part of the transfer, Leighton executives produced a falsely backdated invoice for non-existent services.
On   April 25, 2011, Mr Savage emailed Unaoil from his private Gmail account, stating that Leighton Offshore urgently needed the $5.6 million dollar transaction to occur.
"[Leighton Offshore chief] Russell [Waugh] has been trying to get an agreement signed and we urgently need it signed today," Savage wrote.
Leighton, which changed its name to Cimic in 2015, refused to comment, while Mr Savage denied any involvement in corruption. Mr Cox and Mr Waugh were approached for comment.