LONDON -- British police made two more arrests Wednesday in last week's London subway bombing. Five people are now in custody. Thirty people were hurt when the bomb partially exploded.

Now, Amazon is reviewing procedures after it was revealed its website may unwittingly give tips to would-be bombers.

The main explosive in the London subway bomb didn't go off. Building a functioning bomb is complicated.

But an investigation has revealed that getting ingredients needed too make a simple bomb has become easier than ever. The reason: internet shopping.

CBS News

Look for any of the well-known bomb-making essentials. Sulfur, for example. And under the "frequently bought together" heading, Amazon helpfully suggests you might also want charcoal -- needed for black powder explosive.

Ask for another essential, such as potassium nitrate, and the site suggests sulfur and charcoal.

Dig a little deeper, and battery terminals, presumably for the detonator, and ball bearings -- possibly for shrapnel -- come up.

The algorithms behind these shopping hints may be based on the buying patterns of amateur fireworks-makers and backyard rocket scientists.

But former bomb-disposal officer Chris Hunter says the lists might encourage and help others.

"Those people who are off the radar, I mean radicalized individually," Hunter says. "If an individual isn't a 100 percent sure exactly what components he or she needs, it does give them that missing ingredient and that missing part of the jigsaw."

There's no proof any terrorist bomb has been made from web-bought ingredients, and Amazon says it is now reviewing its practices.

But the web is full of bomb-making tips, including the U.S. Army's own explosives and demolition handbook -- marked down to just over $8.