Mohanad Abdullaahi Goobe was just 15 years old....
He remains in custody for questioning....
Police are now appealing for information....
Investigations are said to continue 'outside of public view'....
A 59-year-old man has been arrested on suspicion of murder....
He was found guilty of manslaughter....
Four arrests have now been made in total....
A murder investigation is now underway...
An arrest has already been made....
