People gather to leave flowers in tribute to victims the day after a truck ran into a crowd at high speed killing scores and injuring more on the Promenade des Anglais, Nice, France, July 15, 2016.ERIC GAILLARD/Reuters
Terrorism appears to be shifting its means of operation from traditional well-planned group actions to more widespread assaults by individuals.
"The real threat now comes from the single individual, the 'lone wolf,' living next door, radicalized on the Internet and plotting strikes in the dark," says Gabriel Weimann, a professor of communication at Haifa University in Israel and a pioneer in the field of lone-wolf terrorism.
"A lone wolf is someone who commits violent acts in support of some group, movement or ideology, but does so alone, outside of any command structure," he explains.
Ted Kaczynski, the so-called Unabomber who operated in the United States from the late 1970s to the early 1990s, was a lone wolf. So was Anders Breivik, the Norwegian who killed 77 people in 2011, and so probably was Mohamed Bouhlel, the man who ran down and killed scores of people on the promenade in Nice on Thursday evening.
Lone wolves, especially those who use vehicles to carry out an attack, are extremely difficult for police and intelligence communities to detect in advance. There are no outwardly visible meetings to plan an operation, no need to acquire weapons or explosives.
That is why groups such as al-Qaeda and the Islamic State now are turning to them.
INTERACTIVE: A guide to the Middle East's web of relationships amid the battle against Islamic State
Since its beginning in 2010, the online magazine Inspire, produced by al-Qaeda in the Arabian Peninsula, has advocated that jihadists living in the West should conduct operations there, rather than travelling abroad and drawing the attention of security officials.
In September, 2014, when the Islamic State was at its height, having swept across much of Iraq and Syria, Abu Mohammed al-Adnani, an IS spokesman, issued chilling instructions to the group's worldwide followers. While many may make their way to the battlegrounds in the Middle East, Mr. al-Adnani called on others to take action in their home countries.
"If you cannot [detonate] a bomb or [fire] a bullet, arrange to meet alone with a French or an American infidel and bash his skull in with a rock, slaughter him with a knife, run him over with your car, throw him off a cliff, strangle him or inject him with poison," he said.
Lone wolves, however, have one weakness: They aren't really alone.
"Behind every lone wolf, there's a virtual [online] pack with which he has been in contact," Prof. Weimann says. They have been informed and inspired by others online, usually on websites connected to terrorist operations.
"Nobody is radicalized by himself," Prof. Weimann says. "All need or use the Internet.
The Netherlands' General Intelligence and Security Service found in a recent study that, while "lone wolves hardly had any contact with like-minded individuals in real life, [they] did maintain active contact with people on the Internet."
That's where counterterrorism forces must focus their attention, Prof. Weimann argues, through very heavy surveillance of the Web and of the so-called Dark Web.
Indeed, that is where governments are doing. But the task is easier said than done. There are thousands of websites connected to terror groups and hundreds of thousands of people are in touch with them.
"Many of the terrorist websites and social media on the surface Web are monitored by counterterrorism agencies and are often shut down or hacked," Prof. Weimann says.
However, such success is only driving the websites underground. Prof. Weimann says groups are moving to the Dark Web, where sites are much harder to detect and where access is gained only through specialized browsers.
Just two days after the deadly attacks on restaurants, a nightclub and a soccer stadium in Paris in November, for example, the Islamic State announced that because of "severe constraints" placed on its sites, the group was closing surface sites and launching a website for the Dark Web.
Already on the Dark Web are channels from al-Qaeda in the Arabian Peninsula, Ansar al-Sharia in Libya, and Jadhat al-Nusra and Jaysh al-Islam in Syria, Prof. Weimann notes.
And their followers are having no trouble finding them. Prof. Weimann points to one authoritative report that within one week's time "one single Islamic State channel went from 5,000 members to well over 10,000."
The growing sophistication of terrorists' use of the Dark Web presents a tough challenge for governments, counterterrorism agencies and security services, Prof. Weimann wrote in a paper published in June. "There is an urgent need to develop new methods and measures for tracking and analyzing terrorist use of the Dark Web."
While terror groups are using the Dark Web for more secure exchanges of information and funds, the aim of most of them is also to reach a wider audience. So for the purposes of encouraging individual acts of terror, they will continue to use the surface Web, making their sites accessible to all, including the intelligence services.