They considered whether they could legally force a suspect's finger or thumb on to the device's fingerprint reader to unlock it, but found they had no such power.
However, they concluded they could stage their own lawful "street robbery" - using a similar snatch technique to a thief - and in June a team set out to do precisely that.
Undercover surveillance officers trailed Yew and waited for him to unlock his phone to make a call - thereby disabling the encryption.
One officer then rushed in to seize the phone from Yew's hand - just as would happen in a criminal mugging. As his colleagues restrained the suspect, the officer continually "swiped" through the phone's screens to prevent it from locking before they had downloaded its data.
"The challenges of pin code access and encryption on some phones make it harder to access evidence in a timely fashion than ever before," said Det Ch Insp Andrew Gould who led the operation.
"Officers had to seize Yew's phone from him in the street. This evidence was crucial to the prosecution."