Reeling from the worst attack on American soil since 9/11, the authorities believe at this stage that Omar Mateen was likely a deranged lone gunman who took the Isis name as a badge rather than a trained operative of the group.

Initial indications suggest that Mr Mateen, who killed 49 at a nightclub in Orlando on Saturday , falls into a distinctive pattern of recent American jihadi violence, a US citizen who operates as a lone wolf and who has little, if any, direct link to a terrorist organisation.

If that conclusion is confirmed by an investigation that is still in its early stages, the Orlando shootings will highlight the different challenge that law enforcement faces in the US compared with Europe, which is battling against organised terrorist cells.