Auditing firm Pricewaterhouse Coopers and Zurich securities law firm Homburger have conducted interviews and scoured scores of emails and other correspondence for evidence that Wauthier was placed under undue stress at work, the two sources said.
"Until now, there is no one event or series of occurrences that would qualify as undue pressure," one person at the Zurich-based insurer told Reuters.
A version of the report has been submitted to Swiss financial regulator Finma, a third source said. It is not clear whether Finma, which declined to comment on Friday, was contributing to the review in any way.
(Read more: Zurich probes if undue pressure caused CFO suicide)
Representatives of Zurich Insurance, Homburger and Ackermann declined to comment. A spokeswoman for Pricewaterhouse Coopers was not immediately available for comment.
Calls to the Wauthier family were not answered.
Sources have told Reuters that Ackermann clashed with Wauthier in the run-up to second-quarter earnings and that in a meeting in mid-August, a day before the release of those results, Ackermann insisted that Wauthier make changes to the results presentation, leading Zurich to signal a lack of progress on business targets.
The review has not uncovered any irregularities in Zurich's finances or reporting, the sources said.
The report has not yet been presented to the board and changes could be made to it before it is finalized, said the source at the company.
Zurich is eager to draw a line under the controversy before November 14, when it reports third-quarter results.
(Read more: Zurich chairman resigns over CFO's apparent suicide)
The company said in August that meeting three-year performance targets for its general insurance and U.S. business Farmers, which include goals for profitability and costs, would be "challenging".
It is due to update investors on the long-term targets on December 5.