As encryption has become more advanced, forces have repeatedly complained that it has hindered their investigations and ability to access incriminating material.

Cameras and motion detectors were added to the site around the bridge to ward off intruders who, officials said, were hindering the investigation.

A report later found that the police were biased in favour of this view, and that this hindered their investigation.

The agency has made allegations of insufficient cooperation from various police services hindering their investigations.

Warren's resignation hindered the investigation.