The Nice Bastille Day killer had likely been preparing his crime for up to a year with the help of several accomplices, the Paris prosecutor announced on Thursday.

One of five suspects - four men and a woman - facing terror charges for complicity returned to the scene minutes after the attack to film emergency services tending to victims, and filmed himself doing so, Francois Molins, the prosecutor confirmed.

The fresh revelations came amid a deepening row over alleged security failings in Nice ahead of last Thursday's attack in which Mohamed Lahouaiej Bouhlel crashed a 19-tonne truck through barriers and into crowds on the promenade des Anglais, killing 84 and injuring more than 300. Some 15 people are still in a critical condition.

Officials had previously suggested that Boulel, who until recently drunk alcohol, ate pork, danced Salsa and had an "unbridled sex life" with men and women, had been radicalised in a matter of days or weeks.

But Mr Molins said fresh evidence from his phone and computer suggested he had likely planned the murderous attack "several months in advance" - even a year.