London is on high alert. After a series of terrorist attacks in the capital, a man from Cardiff is now believed to be responsible for another incident on a group of worshippers near a mosque in London.

Police in the capital have reportedly been handing out leaflets listing what authorities deem as suspicious activity, in the hope that vigilant community members can continue to provide helpful information to law enforcement. Perhaps, in a sign of how online communities play an increased role in radicalization, the leaflet specifically points to use of the dark web as a potential link to terrorism.

"Be aware of what is going on around you—of anything that strikes you as different or unusual, or anyone that you feel is acting suspiciously—it could be someone you know or even someone or something you notice when you are out and about that doesn't feel quite right," another version of the leaflet, which is part of a national campaign and not London specific, reads.

While the list of suspicious activity includes more traditional examples, such as strange transactions on their bank account, or taking photos of security arrangements, one item reads, "is visiting the dark web, or ordering unusual items online."

A section of the leaflet mentioning the dark web. Image: Joseph Cox

In 2015, Islamic State supporters made their own dark web site, although it simply mirrored already available content. A man from the UK was recently jailed for running an extremist-branded site on the Zeronet network, which offered tutorials on how to use hard-drive encryption and other digital security protections.

Got a tip? You can contact this reporter securely on Signal at +44 20 8133 5190, OTR chat at jfcox@jabber.ccc.de , or email joseph.cox@vice.com

But much of the communication between Islamic State supporters takes place on social media, such as Telegram. And the group's and supporters' propaganda videos are often distributed on everyday social network sites. A Motherboard investigation published on Monday found that YouTube is regularly leaving pro-Islamic State videos online for days and weeks.

Far-right groups also make extensive use of web-forums and social media. According to The Guardian, a UK far-right organization regularly uses the dark web.

"Your call could save lives," the leaflet adds.