The link between the Islamic State extremist group and a recent string of terrorist attacks in Europe and the United States is weak, said Europol on Wednesday, noting that mental illness might very well be the motivation for such lone wolf acts.

The special report on lone actor attacks - which focuses on attacks in Orlando, Florida; Magnaville, France; Nice, France; and Wuerzburg, Germany - is an addendum to the EU police agency's annual terrorism and trends report, details of which were released in May.

Europol noted that some of the individuals behind the attacks had claimed allegiance to Islamic State - which had previously urged Muslims living in Western countries to launch attacks.

However, it also pointed out that, even though lone wolf attacks are a favourite of groups like al-Qaeda and Islamic State, it is too early to assume these aren't attacks that would have happened anyway for which groups are claiming credit after the fact.

"Although IS has claimed responsibility for the latest attacks, none of the four attacks seem to have been planned, logistically supported, or executed directly by IS, according to the information available to Europol," the agency wrote, using a common acronym for the group.

For example, while the attackers in Orlando, Magnaville and Wuerzburg all claimed to be Islamic State supporters, "their actual involvement in the group cannot be established," read the statement.

In the case of Nice, Europol said there is no evidence the attacker considered himself to be an Islamic State member, though the group has claimed him.

"IS has endorsed the attacks, but the perpetrators' affiliation with the group has not been clear."

Europol also noted that Islamic State's language in claiming the attacks was very specific.

When Islamic State members attacked Paris in November and Brussels in March, the group's propaganda stated clearly it had sent its members to perpetrate attacks. However, in the more recent incidents, Islamic State-controlled media have cited unidentified sources telling them that attacks were carried out by group members.

"This differentiation may indicate that IS would like to maintain a level of 'reliability,' should information contradicting its claim to responsibility emerge."

Furthermore, Europol noted that around 35 per cent of individuals who launched lone wolf attacks between 2000 and 2015 suffered from some form of mental disorder, implying that some of these people might have committed crimes anyways, but chose to link them to terrorist groups to achieve greater meaning.

Europol noted that the Nice attacker was receiving treatment for mental health problems and that his actions mirrored a pair of incidents in France in 2014 in which people drove vehicles into crowds. Both of those were attributed to mental health matters.

"Despite the fact that a number of lone actors attach religion and ideology to their acts, the role of potential mental health issues should not be overlooked," read the report.

But that does not make the Islamist propaganda less dangerous, Europol noted.

"Even though ideology may be used by terrorist perpetrators to cast a shadow over the deeper individual/pscyhological motives of their acts, one should not disregard the motivating power of the jihadist discourse to certain audiences," read the report.

"In cases where the perpetrator has a mental disorder, ideology might have an aggravating effect, leading to different target choices and scaling of the attack."

Omar Mateen killed 49 people when he stormed Pulse, a gay nightclub, in Orlando on June 12, before dying in an exchange of gunfire with officers. In the June 13 Magnaville attack, Larossi Abballa killed a policeman and his wife before being killed by responding officers.

Mohamed Lahouaiej Bouhlel killed 84 people when he ploughed them down with his truck during Bastille Day celebrations on Friday before he was shot dead by police. On Monday, a still-unidentified 17-year-old rampaged through a train in southern Germany, injuring five, before he was shot dead.

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