A new video has emerged showing the twin towers on the morning of 11 September, 2001, shortly after they were attacked.

The footage, which has never been seen before, is shot from a New York Police Department's air and sea rescue helicopter.

The video was released after the National Institute of Standards and Technology received a request under the Freedom of Information Act.

Only police helicopters were allowed in the airspace near the World Trade Centre after a total air exclusion zone was declared.

Gordon Corera reports.