Accounting watchdogs at the Financial Reporting Council have widened their probe into the auditing of Carillion in the run-up to its collapse.
FRC chiefs will now be looking into the books for 2013 as well as the current investigations into auditing between 2014-17.
The watchdog is also investigating the conduct of former group finance directors Richard Adam and Zafar Khan.
In an update last month the FRC said: “A key area of focus has been the financial performance of Carillion’s major contracts in both the construction and services divisions, and whether Carillion management and its auditors ensured that this was appropriately reported in its financial statements.
“The investigations are also considering conduct relating to pension liabilities, goodwill, cash disclosures and going concern.
“The FRC has obtained and is analysing very significant quantities of documents relating to these areas.
“Detailed interviews have been conducted with audit team members and Carillion senior executives and further interviews are planned for early 2019.
“The FRC meets regularly with case teams from other agencies to identify issues of common interest and ensure that relevant documents and information are shared to the extent permissible.
“The matters being investigated by the FRC are complex and are expected to continue well into 2019.
“They are a critical stage in the process, the findings of which form the bedrock on which any regulatory proceedings are based. The FRC is committed to completing its investigations promptly and thoroughly.”