Is Amazon recommending bomb ingredients?
Amazon is to "review" how products are presented on its website, following revelations about bomb-making ingredients for sale.
A Channel 4 investigation found the retailer's algorithms grouped together household ingredients that when combined could make a lethal bomb.
Less than a week ago, a home-made explosive device was detonated on a London Underground train.
Amazon said it now intended to display products "in an appropriate manner".
What do we know?
The Channel 4 investigators put together a shopping basket of goods that could have theoretically been used to create a deadly explosion. One of the ingredients was said to be widely used in food production, although it was not identified.
The reporters discovered that when some of the bomb-making ingredients were selected, Amazon's algorithms suggested others via a "frequently bought together" panel on its pages.
The journalists also found that remote detonators, ignition systems and ball-bearings - which could be used as shrapnel - were also promoted in a separate "customers also bought" section.
Such ingredients are not illegal to buy individually. But the programme pointed out that people had previously been arrested for buying large quantities of the chemicals involved.
The Amazon shopping basket created by Channel 4 had 45kg (99lb) of ingredients although the transaction was not completed. Under current legislation, an individual can produce 100g of gun-powder for private use.
Are hundreds of people buying bomb-making equipment online?
The fact that the bomb-making ingredients popped up in the promoted panels has led many to speculate that others must have purchased the goods together.
The BBC asked Amazon how many people would have needed to have purchased the ingredients before they were grouped in these ways, but received no reply.
But according to ex-Amazon software engineer Owen Miller, there would have to had been a "significant number".
"If a few people bought post-it notes and blue shoes that wouldn't show up as recommended," he explained.
"It is defined on probability, so it is difficult to say exactly but it would be significant."
Calum Chace, author of The Economic Singularity, said it was unsurprising that Amazon was remaining tight-lipped on the inner-workings of its algorithms.
"These algorithms are a closely-guarded secret," he said.
"But it is worth noting that one third of all purchase searches start at Amazon not Google, so there could be a lot of people speculatively playing around with such product combinations."
Many Twitter users remarked that a bomb-maker would "have to be pretty dumb" to acquire such ingredients via Amazon, because their purchase would be far easier to trace than someone walking into a high street store.
It should also be pointed out that bomb-making recipes can be found elsewhere online, and it seems unlikely a terrorist would rely on Amazon's recommendations.
In 2013, the Australian government published a list of 96 ingredients that could be used for improvised explosive devices.
"If you think that I'm being irresponsible in giving away bomb-making instructions to terrorists, I can assure you that unfortunately these instructions are already easily accessible via the internet," federal attorney-general Mark Dreyfus said at the time.
There have also been plenty of offline guides, including the Anarchist's Cookbook, which caused a furore when it was published in 1971.
What is Amazon doing about it?
The investigation has sparked outrage on social media with many calling for the US store to alter its algorithms to stop grouping bomb-making ingredients.
"All products sold on Amazon must adhere to our selling guidelines and we only sell products that comply with UK laws," the company told the BBC.
"In light of recent events, we are reviewing our website to ensure that all these products are presented in an appropriate manner.
"We also continue to work closely with police and law enforcement agencies when circumstances arise where we can assist their investigations."
It did not explain how it planned to do this.
Mr Miller told the BBC that recommendations could be filtered.
He also mentioned that that "a blacklist of keywords" already existed within Amazon, although he added that in this case any banned words would have to be specifically related to bombing.
What are governments around the world doing to limit online information about bomb-making?
In 1994, US Congress passed a law making it illegal to distribute or pass out information about building bombs. However, the law only applies when the information is passed out to help someone commit a federal crime - meaning the website would also have to incite readers to build such a bomb.
In 2004, German authorities forced a 17-year-old to shut down his website, entitled Der Abarisch Sturm after he posted detailed instructions of how to build a bomb.
In 2007, EU security commissioner Franco Frattini outlined a set of anti-terror proposals, including plans to criminalise the publication of bomb-making instructions on the net. The legislation never became law.
Mr Miller told the BBC that if people were determined to buy bomb-making ingredients, it might "be better" they did so via Amazon.
"The trend data from Amazon will enable [the authorities] to learn when others are up to no good," he said.