A 34-year-old man has been arrested on suspicion of murder and attempted murder in connection with the killing of nine-year-old Olivia Pratt-Korbel, Merseyside Police said.
The force said the man remains in custody and is being questioned by detectives.
Meanwhile, a 41-year-old man from Knowsley has been arrested on suspicion of assisting an offender and is also in custody.
The men were arrested in the Runcorn area in the early hours of Sunday.
Later on Sunday afternoon, Merseyside Police said officers executed warrants at two addresses in the Liverpool area and arrested a 29-year-old man on suspicion of assisting an offender.
He is in police custody where he is being questioned by detectives.
Olivia died after being shot in her home in the Dovecot area of Liverpool when a gunman chased convicted burglar Joseph Nee into the property at about 10pm on Monday 22 August.
The cause of death was a gunshot wound to the chest, a post-mortem examination found.
Her mother Cheryl was also injured.
The shooting happened 15 years to the day since the gun murder of schoolboy Rhys Jones in Liverpool.
Neither of the men who entered Olivia's home had any links to her family.
Providing the first update on Sunday, Detective Chief Superintendent and head of investigations Mark Kameen said: "I continue to urge anyone who has information that can help our investigation into Olivia's tragic murder to please come forward so we can bring those responsible to justice.
"A number of people have been arrested in respect of this investigation. However, we still need the public's help in ensuring that we can a build a strong evidential picture so justice is served for Olivia and her family.
"With this, I appeal for those who may have information, no matter how small, that could continue to help our inquiries to contact us directly or anonymously and we will do the rest."
A second update said: "Detectives have this afternoon, Sunday 4 September, arrested a further suspect in connection with the murder of nine-year-old Olivia Pratt-Korbel.
"Officers executed warrants at two addresses in the Liverpool area and arrested a 29-year-old man on suspicion of assisting an offender.
"He is in police custody where he is being questioned by detectives."
A 36-year-old man from Huyton and a 33-year-old from Dovecot who were arrested on suspicion of murder and two counts of attempted murder in relation to Olivia's death have been released on bail.
On Tuesday, an inquest heard of how Olivia was scooped up by police and taken to hospital after being shot, as an officer tried to save her by using his hand to cover the chest wound.
The church next to Olivia's school held a vigil for her on Wednesday.
A minute's applause in memory of the nine-year-old was also held during the ninth minute of play at the Merseyside derby match between Liverpool and Everton on Saturday.
A service will be held for Olivia in Liverpool on 15 September, according to a funeral director's notice.
"Family requested everyone to wear a splash of pink," the message on the website of Debbie Bennett Funeral Directors said.
"Olivia will be sadly missed by all her family and friends.
"Funeral Mass at St Margaret Mary's Church, Pilch Lane, Dovecot on 15 September at 11am, followed by a private burial."