The Hallmark of an Improvised Explosive Device ...

... is that it's improvised ("manage[d] in a makeshift way; do[ne] with whatever is at hand"), as opposed to mass-produced from standardized parts.

The hallmark of an IED is not that it has ball bearings in it.

"One piece of evidence I'm particularly interested in is whether these devices contained ball bearings, because that's the signature of an IED," [House Homeland Security Chairman Michael McCaul, R-Texas] said. IEDs, or improvised explosive devices, are frequently used by al-Qaeda-linked groups and would suggest a foreign link, he said.

And pretty much any and every bomb used by criminals or terrorists, working individually or in groups, is by definition an IED. They don't go buy an Acme Model I bomb, they find whatever materials they can find and build themselves something out of those materials.

Might an IED have ball bearings in it? Sure. But so do some manufactured bombs like the M18 Claymore.

Might al Qaeda use ball bearings in an IED? Sure. But so might any of the remaining dissident Irish Republican Army groupings, the Ku Klux Klan, or the White Bulger Fan Club.

While it's possible that there might be "signature elements" to the Boston Marathon devices, use of ball bearings wouldn't per se be an element that would narrow anything down much.

To put a finer point on it, McCaul is a dumbass.