Two robberies are believed to have been carried out by a group of up to 10 teenage boys.
On May 31 and June 14 two teenagers were robbed of their mobile phones and wallets in Roding Road and Felsted Road, Loughton, between 9pm and 10pm.
A 16-year-old from Chigwell, a 14-year-old from east London and a 14-year-old from Barking were arrested on suspicion of robbery and bailed until August 18.
Officers believe the group may have used public transport to travel from London to Loughton.
Anyone who saw large groups of teenage boys in Loughton on the nights in question can contact Dc Stephen Baker at Loughton CID on 101 or Crimestoppers on 0800 555 111.