Twenty-two people are dead, including children, after the suspected terror attack.
The man who killed 22 people in a suicide attack had recently returned to the city from Libya.
The security service is to examine its actions after tip-offs from the public about extremist views.
Justin Bieber, Coldplay and Katy Perry will perform alongside the US singer on Sunday.
The monarch says she is shocked by the targeting of children, as she visits the injured in hospital.