Avon and Somerset Police chief Nick Gargan in data protection probe
A chief constable being investigated for gross misconduct will also be interviewed on suspicion of data protection offences.
Avon and Somerset chief Nick Gargan was suspended over allegations he made inappropriate advances to female staff.
The police watchdog said he may have breached the Data Protection Act by sending emails with personal data to individuals unconnected to the force.
Mr Gargan, who denies the charges, is due to be interviewed soon.
Rachel Cerfontyne, of the Independent Police Complaints Commission (IPCC), said: "Mr Gargan has now been served with a notice informing him that this aspect of his conduct is subject to a criminal investigation."
Investigators have "examined a large amount of computer and telephone data" during the probe, with 50 members of staff from the force providing "information and intelligence", said the IPCC.
Speaking on behalf of Mr Gargan, a spokesman for the Chief Police Officers' Staff Association said Mr Gargan would "continue to cooperate" with the investigation.
"Chief Constable Gargan is pleased that progress has been made in relation to the allegation concerning inappropriate behaviour and that this allegation is no longer being treated as a criminal matter," the statement continued.
"He is keen to return to work and hopes that the investigation can be progressed expediently."