Carillion, the UK's second-biggest construction firm, is being examined by the financial watchdog.
During that period the HS2 contractor's share price plunged and its chief executive departed following a series of profit warnings.
Carillion said it is co-operating fully with the FCA.
In July 2017 Carillion said it was writing off £845m in revenue it had expected from several large contracts, and it admitted that its already big borrowings were becoming bigger.
The profit warning, which resulted in the departure of chief executive Richard Howson, sent the company's shares down by about 40%.