Paris, July 16 (Reuters): As authorities investigate the motives for a mass killing claimed by the Islamic State in Nice late on Thursday, analysts say the case appears to highlight a shift in the profile of those launching attacks in the name of hardline Islamist groups.

Mohamed Lahouaiej Bouhlel, who killed at least 84 people by driving a truck through crowds in the French town, was not a pious, educated man in the mould of Mohamed Atta, one of the hijackers behind the 9-11 attacks in the US in 2001.

Rather, neighbours and family describe him as a troubled man who lived apart from his wife and three children and drank alcohol, something forbidden by Islam.

Bouhlel's sister said he spent years seeing psychologists before leaving Tunisia for France in 2005. "My brother had psychological problems, and we have given the police documents showing that he had been seeing psychologists for several years," Rabeb Bouhlel told Reuters.

Bouhlel's father told French television that the family had sought medical treatment after his son had a breakdown.

"He had psychological problems that caused a nervous breakdown; he would become angry, shout, break everything around him," Mohamed Mondher Lahouaiej Bouhlel told the TF1 and France 2 channels."We had to take him to the doctor."

"It seems that he (Bouhlel) was radicalised very quickly," said French interior minister Bernard Cazeneuve.

That poses a big problem for authorities, who have put much of their focus on tackling hardline Islamist ideology by seeking to spread counter-arguments in schools and mosques.

Tunisia-born Bouhlel, who was shot dead by police, had had several run-ins with the law, including a March 2016 conviction for hurling a wooden pallet at a driver in a road rage incident. His sister also told Reuters he saw psychologists for several years before he left Tunisia in 2005.

His case echoes that of Omar Mateen, who killed 49 people at an Orlando night club in June in the name of IS, the deadliest mass shooting in modern US history.

Mateen had a troubled youth, was disciplined often at school and, before carrying out his attack, researched medication to treat psychosis, a relative told Reuters.

"Islamic State is an organisation which attracts a very broad variety of followers, from the most convinced, to the most adventurous, to the most unstable or psychotic," said Prof. Rik Coolsaet, a terrorism expert linked to Ghent University in Belgium. Several of the men involved in apparently IS-inspired attacks in Paris in November 2015 and in Brussels in March 2016 also had a history of crime.