The authorities are now looking for anything that might have been part of the explosive devices - bits of metal or sharp objects that they might be able to trace back to its owner or owners.

BBC: Forensics investigators decode the Boston bombings

In this room was a gas heater that had an improvised exhaust system attached to it made from bits of sheet metal hammered together.

CNN: A visit to Osama bin Laden's lair

In most auto factories, the leftover steel cutouts are stacked up, then sold to a foundry, where they are melted with other bits of steel and converted into scrap metal.

FORBES: How GM Makes $1 Billion A Year By Recycling Waste