Hackers last week downloaded 400GB of data from the firm, which makes software that allows law enforcement and intelligence agencies to tap into the phones and computers of suspects.
Much of the data, including thousands of private corporate e-mails, has since been dumped onto the WikiLeaks website.
The source code of a number of its top-secret programmes has also been published online, the company has said, meaning that "terrorists, extortionists and others can deploy this technology at will if they have the technical ability to do so".
Investigative sources said the six suspects had already been placed under investigation in a separate case for allegedly revealing the company's industrial secrets.
That probe was launched after Hacking Team chief executive David Vincenzetti filed a complaint in May accusing the six former employees of having revealed part of the company's source code, according to the sources.
The two investigations have now been combined.
A Hacking Team spokesman did not immediately respond to an e-mail requesting comment.
The leaked e-mails show that Hacking Team worked with numerous state institutions in an array of countries, including Italy, the United States and Australia. Its customers include the US Federal Bureau of Investigation, according to internal documents published online.
It also had dealings with countries criticised for their human rights records, such as Libya, Egypt, Ethiopia, Kazakhstan, Morocco, Nigeria, Saudi Arabia and Sudan.
Vincenzetti has previously said a government might have been behind the hacking of the company's systems. — Reuters
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