At first glance, there was no witness to the grisly murder of Christine Sullivan, who was stabbed to death alongside a friend, Jenna Pellegrini, at her home in rural New Hampshire in the U.S. last year.

But as detectives searched the secluded colonial-style home and its tree-lined garden, they came to realise that a big clue was staring them in the face.

For on the kitchen worktop was an Amazon Echo, one of the increasingly ubiquitous ‘smart speakers’ that can perform a host of household tasks, from checking the weather and playing music to creating shopping lists and looking up recipes.

Such devices follow a user’s spoken commands, so are almost always listening out for fresh orders. What’s more, this one was likely to have been within earshot of Sullivan’s cold-blooded killing, and could be the first-hand witness police were looking for.

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