Detectives in Foyle are investigating a report of a suspicious approach to a child in the Claudy area yesterday evening, the third such incident in recent weeks.
Police do not believe these three incidents are linked however they are keeping an open mind.
In each case, very different descriptions of the males involved were reported.
At around 9.30pm, two males entered a shop at Blucher Street, one was armed with a knife and the other was armed with a hammer.
They threatened the shop assistant and demanded money before they fled with a sum of cash.
The female shop assistant was not injured but was badly shaken.
The males are described as being around 6ft tall and thin build. One was wearing light grey coloured tracksuit bottoms, a light grey coloured hoodie top and and a black balaclava.
The other was wearing dark bottoms, a dark blue coloured hoodie top, and a black and white coloured football scarf.
Police are asking anyone who may have seen the males in the area of Blucher Street between 9pm and 10pm to contact them at Strand Road station