Prosecutors placed them under investigation on charges of abusive computer system access and revealing industrial secrets, Ansa reported.
The six were reported by company founder David Vincenzetti in May for allegedly accessing proprietary software development code.
The probe comes a week after the firm announced that it had more than 400 gigabytes of data breached and leaked.
Investigators are looking into whether there is a connection between the ex-employees and the hacking attack.
The hackers allegedly posted internal documents, defaced the company's logo and made public embarrassing details of the firm's commercial dealings. The attack was a huge loss of face for the firm, which was set up in 2003.
HackingTeam sells the Da Vinci malware surveillance software to law enforcement agencies, according to media reports, although it claims to only deal with ethical governments.
It has been marked as an Enemy of the Internet by media rights watchdog Reporters Without Borders.
According to the leaked documents, the company's clients include outfits based in countries such as Sudan, Kazakhstan, Bahrain and Hungary.
The leaked records reportedly showed the company had commercial transactions with companies and governments which used the malware to spy on journalists and political opponents.
HackingTeam said in a statement last week that virtually all clients had suspended use of the Remote Control System (RCS) that was compromised in the attack.
“This is an important step to protect on-going police and intelligence investigations,” the company said.
“There have been reports that HackingTeam has 'backdoors' in its systems that would allow us to control the systems remotely. This is simply not true. Clients operate our technology on their own computer systems, and so it is clients who must take action to suspend operations.”
The firm added that because of the hacking attack, the ability to control who uses the technology has been lost.
“Terrorists, extortionists and others can deploy this technology at will if they have the technical ability to do so. We believe this is an extremely dangerous situation.”