The four male and one female officers in Frankfurt have been suspended from their duties during an ongoing investigation.
The group was discovered after allegedly threatening to "slaughter" the lawyer's two-year-old daughter in an anonymous fax message, reported the Frankfurter Neue Presse daily.
The message called the lawyer, Seda Basay-Yildiz, who has represented suspected Islamist militants, a "lousy Turkish swine" and told her to "piss off while you still get out of here alive".
The message, which she received in early August, was signed "NSU 2.0" - a reference to neo-Nazi terror cell the NSU, or National Socialist Underground, that murdered eight Turkish immigrants.
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Basay-Yildiz, who has also represented families of NSU victims, said she routinely receives threats but was baffled that this one contained the name of her daughter and her private address.
She reported the fax to police, sparking an investigation which indicated that officers had accessed her personal data on a Frankfurt station computer, reported the daily.
The probe sparked raids on the officers' homes and the confiscation of computer hard-drives and cellphones, which reportedly led to the discovery of the WhatsApp group.