Two boys were left with head and facial injuries after a fight between a group of teenagers in Milton Keynes last week.
A man is believed to have witnessed the altercation at around 8pm on Tuesday near the shops in Great Linford.
Officers are keen to track down this person.
Two boys aged 12 and 13 years old have been arrested on suspicion of assaulting a person thereby occasioning them actual bodily harm.
They have been released on bail until October 1.
Anyone with information is asked to contact investigating officer PC Samantha Gregson on 101.