The veracity of the images could not be confirmed, and there was considerable suspicion that they could have been altered or falsified. A representative of Facebook, the parent company of WhatsApp, declined to comment on the matter, pointing only to an article in the Swedish press that suggested the images were forgeries.

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In the unconfirmed images, the alleged attacker appeared to correspond with a supporter of the Islamic State from Tajikistan. In Russian, the man appeared to ask for advice on how to build a bomb, before later detailing his plan to run over pedestrians in a motor vehicle. After the attack, the man allegedly wrote: “I ran over ten people in the center of Stockholm, now I have to get out of here.”

The image also appeared to reveal that the suspect sent a video of himself at the scene of the attack.

Despite the doubts about their veracity, the images have been widely reported on and have sparked widespread discussion around the world. If accurate, they raise serious questions about the Islamic State's role in the planning or aiding of the Stockholm attack, but also about issues surrounding the encryption of data on messaging services such as WhatsApp.

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However, experts could not confirm the authenticity of the messages — and there were major questions raised Monday about their credibility. The images were first shared Friday by the Russia-based social media account TVJihad, not long after the attack in Stockholm took place.

The TVJihad account claims to be a joint project of the Kavkaz Center, a website that focuses on Chechen news, and the Russian independent television channel TV-Dozhd. Mikael Storsjo, a Finnish entrepreneur who is the registered owner of Kavkaz Center, told the Dagens Nyheter newspaper that his website had no links to TVJihad, and suggested that it was likely that the owners of the account had created the conversation shown in the images.

TVJihad appears to often post content that parodies or mocks Islamist extremist groups, though occasionally it intersperses more serious-seeming information. A number of experts suggested that the account was rumored to have been founded by members of a youth movement that supports the Kremlin known as Nashi, though the information could not be confirmed.

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In the aftermath of last month's Islamic State-claimed attack on London, British officials pushed WhatsApp and other messaging services for greater access to data in cases involving terrorism. WhatsApp, like some other apps, uses end-to-end encryption for messages so that they can be read only by their intended recipient. In theory, this means that companies cannot read the messages themselves and cannot pass them on to law enforcement if requested to.

Pavel Durov, the founder of a Russian messaging app called Telegram that uses similar encryption, has admitted that such measures do sometimes allow terrorists to communicate. “There’s little you can do, because if you allow this tool to be used for good, there will always be some people who would misuse it,” Durov told CBS last year. Requests to Telegram for comment on the images were not returned.

Thomas Hegghammer, a research fellow at the Norwegian Defense Research Establishment (FFI) and an expert on extremism, suggested that if the images were real, they showed how little would-be terrorists cared about their messages being read. “To me these screenshots are yet another indication that jihadis no longer fear surveillance the way they used to,” Hegghammer said in an email.

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On Monday, TVJihad posted a number of messages on Twitter and Telegram, pushing against the claims that the account was linked to “Russian nationalists” and claiming that other information had corroborated the information in the images. In the end, this information would be proved right, TVJihad implied.

“The best proof that the correspondence is not fake is that in one or two days will publish video from an Uzbek,” the account wrote.