Some staff who have worked on the operation told ITV News anonymously they have witnessed security breaches by G4S staff already working at Olympic venues.
Among the key claims they make are:
- Cargo areas of lorries and contractors vans have not been adequately searched by either dogs, scanners or other security teams.
- In some instances dogs which were not trained to find explosives were used to carry out fake searches of vehicles
- Some assessments of dogs and handlers have been faked
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Detectives have charged 19-year-old Bilal Mumin with murder, grievous bodily harm and possession of an offensive weapon.
Members of the RMT on Arriva Rail North (Northern) walked out for 24 hours, with more stoppages due on Thursday and Saturday.
Police said a dozen shots were fired, but no arrests have been made and they are looking for a dark vehicle.