Three men carrying what police believe was a stun gun robbed two people of their mobile phones.
The men approached a boy in the park area of Olympic Way on the Queensway estate in Wellingborough on Friday night and took his Blackberry phone.
It is thought the same group of men then approached a man in Kilborn Road and took his Blackberry phone.
The two linked incidents happened between 8pm and 9.30pm.
The group was made up of two white men and one black man.
One of the white men was carrying what appeared to be a stun gun.
The black man was around 6ft 5ins tall and was wearing a balaclava which covered his face.
Detective Constable Michael Ringrose, of Wellingborough CID, said patrols had been carried out in the area since the incidents.
He said: “The offenders were wearing dark clothing and face coverings on what was a very cold night. We believe one of the offenders may have been carrying a stun gun because sparks were seen to come from an implement that one of them was carrying.
“In both cases, Blackberry phones were stolen and we would advise young people who have these phones not to flash them around and to be aware of what is going on around them at all times.
“Safer Community Team officers will be out patrolling the Queensway estate and we would appeal to anyone with information about these robberies, or who noticed any suspicious activity around Queensway on Friday evening or night, to contact us.”
Denise Barnett, who lives on the Queensway estate, said: “It’s quite distressing that people can get hold of those things.
“It was -11C on Friday night in Wellingborough so it’s absolute madness they were out doing this.”
Witnesses or anyone with information are asked to contact Northamptonshire Police on 101.
Information can also be given anonymously to Crimestoppers on 0800 555111.