Originally Posted by flourish
Okay, so as far as I can tell, the bra came apart at the seam, which is why the threads are still hanging on to that piece. Now, not having been able to look at the bra in "real life," or see a super detailed photo of the part where it came apart, it's difficult to say, but it appears, to me, that it was torn, ripped, or pulled apart from the other piece. However, despite what Hollywood and bad romance novels would like you to believe, it is actually quite difficult to rip through the materials in many, if not most, female undergarments. It of course depends on what exactly every piece of the bar was made of, as you have your various elastics, lycra, spandex, and other stretchy materials, fiber blends, double layers, reinforced panels, padding, plastic and/or metal under-wires/stays, and so forth.
It is possible that a knife was used to make a small cut to help the bra be torn off easier, but my best guess, using my years of experience shopping for and wearing bras, as well as the visual inspection of the photos of the bra, is that the bra was likely torn off with bare hands, which would require a significant amount of force.
If anyone feels the need to experiment, you can buy cheap bras at Goodwill and try ripping them apart (not even necessarily off of someone and you'll see how difficult it could be (of course depending on fabric, how many times bra was washed, whether it was hand-washed or machine-washed, and whether it was hung to dry or dried in a dryer--a dryer-dried bra's elastic will weaken quicker than one that has been hung to dry)
FOR WHAT IT'S WORTH