Police are investigating two recent robberies that took place in Hemel Hempstead on April 5.
Detectives are appealing for information after two incidents at The Marlowes Shopping Centre between 2-4pm on Tuesday.
Just after 2pm, a 14-year-old boy was allegedly threatened into handing over his phone behind Waterstones.
Shortly afterwards at 3.30pm, a 16-year-old boy was allegedly threatened into handing over his wallet.
No one was hurt in the incidents, which are being treated as linked.
A 15-year-old boy from Hemel Hempstead has been arrested on suspicion of robbery, possession of a bladed article and possession of Class A. He has been released on police bail while enquiries continue.
Police continue to investigate and are asking for anyone with information to get in touch.