The Nice attacker sent a picture of himself laughing amongst the Bastille Day crowds before carrying out mass murder, according to his brother.
Hours later Mohamed Lahouaiej Bouhlel drove a 19-tonne lorry down the promenade killing 84 people and injuring 200 more, many of them children.
His brother Jabeur told Reuters Bouhlel called him for a final time on Thursday afternoon - just hours before the attack.
"That last day he said he was in Nice with his European friends to celebrate the national holiday," Jabeur said.
Jabeur has not supplied the photo he claims Bouhlel also sent him but said in it his brother seemed very happy and pleased, he was laughing a lot."
It has also emerged that moments before the attack Bouhlel sent chilling text messages detailing his plans.
One read: "Bring more weapons. Bringing in 5," according to judicial sources quoted by French TV.
An earlier text read: "It's good. I have the equipment."
Bouhlel had a criminal record and was known to French police, but not to the intelligence services. His motive remains unclear.
His brother told Reuters Bouhlel had spoken about returning home to Tunisia soon.
Bouhlel's sister has said he was treated for psychological issues for years before leaving Tunisia.
A psychiatrist who treated him more than a decade ago told Reuters he had been aggressive towards his parents and had body image problems.
Dr Chemceddine Hamouda said: "Sometimes he had tried to lock his parents in a room in their house."