A further four people were detained today on suspicion of the murder of 11-year-old Rhys Jones, police said.
The youngster was shot in the neck last week as he walked home from football practice in Croxteth, Liverpool.
Two men, aged 20 and 24, and two boys, aged 17 and 16, all from the Liverpool area, were detained this morning.
They will be questioned by Merseyside Police later today.
Earlier today, detectives investigating Rhys's murder urged a mystery caller to get in touch with them.
Police said a witness who called the Crimestoppers hotline on August 24 should ring officers trying to piece together the circumstances of the fatal shooting.
The caller reported seeing a male on a mobile phone, a spokesman said this morning, describing the information as "significant".
Detective Superintendent Dave Kelly said: "I would appeal for this person to make direct contact with me.
"We appreciate the information they have supplied but we now want to speak to them directly.
"There's a lot we can do to protect a witness's identity and we will use every appropriate measure.
"The information from Crimestoppers was significant but we now need to speak to the caller directly."
Everton-mad Rhys died last Wednesday evening after being shot through the neck in the car park of the Fir Tree pub in Croxteth, Liverpool.
His mother, Melanie, 41, was one of the first to arrive at the scene. She held and comforted him as he lay dying.
Rhys was rushed to Alder Hey Children's Hospital but was pronounced dead soon after.
The cold-blooded murder - as Rhys was making his way home from football training - sparked worldwide revulsion and a national debate on youth and gun crime in Britain.
Before today's arrests, police had arrested 11 teenagers in connection with the killing. Seven were released on police bail and four released without charge.
Appeals for information have been made at Everton Football Club's Goodison Park ground and Liverpool FC's Anfield Stadium, where Rhys's family have been applauded by fans united in their grief.
The youngster's funeral arrangements are expected to be disclosed today.
His mother and father, Stephen, 44, are understood to be finalising plans with Merseyside Police for the funeral, expected next week.
Anyone with information is asked to call the incident room on 0151 777 8722 or to text CRIME to 61051.
Merseyside Police described today's arrests as "new", bringing the total number of people held in the investigation to 15.
Assistant Chief Constable Patricia Gallan said: "Detectives have this morning arrested a further four people in connection with the ongoing investigation into the death of 11-year-old Rhys Jones.
"Arrests made so far are based on consistent information from callers and round-the-clock inquiries by detectives.
"Whilst this inquiry progresses swiftly, it is very much part of a focused, clear and confident investigation strategy. All individuals arrested so far are absolutely critical to the inquiry.
"People are aware that we have described one individual firing a gun but, as with any offence, there are various stages and elements to it, such as where the gun came from and what occurred afterwards.
"When we have grounds to suspect people involved in any stage, they will be arrested in connection with this offence."Reuse content