Flags fly at half-mast as an armed British Police officers walks on patrol with his weapon outside of Greater Manchester Police force's headquarters in Manchester, northwest England on May 23, 2017, during a visit by Britain's Prime Minister Theresa May. Twenty two people have been killed and dozens injured in Britain's deadliest terror attack in over a decade after a suspected suicide bomber targeted fans leaving a concert of US singer Ariana Grande in Manchester on May 22. / AFP PHOTO / Oli SCARFF