McAfee claims that evidence from the data dump that included all of Ashley Madison's users show that the information was stolen by someone inside the office, perpetrated by a woman.
Adultery website Ashley Madison that encourages married couples to indulge in secret affairs was hacked on July 20.
'"Life is short. Have an affair" is the tagline of the website.
According to news reports, hackers stole 37 million personal records from the site and threatened to release them if the website does not shut down "permanently in all forms."
"From the data that was released, it was clear that the perpetrator had intimate knowledge of the technology stack of the company (all the programs being used)," McAfee wrote.
The claims correspond with experts' analysis of previous major cyberattacks. The Sony Hack, for instance, has been said to have likely originated with a disgruntled ex-employee with access to the company's computers, taking the information as revenge or for personal gain.