The Financial Reporting Council (FRC) announced its enforcement division will now analyse financial statements by Thomas Cook's auditors EY stretching back to September 2017.
Thomas Cook, the world's oldest travel firm, collapsed in September after failing to produce a funding lifeline, affecting 9,000 jobs.
The watchdog said it would now extend its inquiries by a further year, more than two months after starting the process, but did not reveal any of its current findings.
Business secretary Andrea Leadsom said: "There are serious questions to be answered about the way in which Thomas Cook was managed and audited in the period prior to its collapse, and so I welcome that the FRC are going to look into the years preceding 2019."
EY, one of the UK's big four auditors, started completing Thomas Cook's audit work in 2017 after taking over from rival PwC.
In May, Thomas Cook reported a £1.5 billion half-year loss and issued its third profit warning in a year in a set of accounts, which also saw EY warn that there was "significant doubt" over whether the travel firm could continue as a going concern.
In a statement, the FRC said: "The matters being investigated by the Financial Reporting Council concerning EY's audit of the financial statements of Thomas Cook Group for the year ended 30 September 2018 will now include an investigation into EY's audit of Thomas Cook's financial statements for the year ended 30 September 2017.
"The FRC will continue to keep the scope of the investigations under close review."
The FRC aims to complete all of its investigations within two years and has the power to take EY to tribunal if it finds evidence of wrongdoing.