Apparently, the problem was caused by metal particles that contaminated the batteries' Lithium-Ion cells. As a result of the talks with Dell, Sony beefed up its battery design with thicker cell linings to prevent metal particles from getting in. However, the recall was only initiated once the flaw was "clearly linked" to Dell notebooks catching fire. "We didn't have confirmation of incidents [involving fires] until relatively recently. We received reports, but didn't know if there were environmental situations not related to the systems themselves," the Sony spokesman told InfoWorld.