According to a source for the Wikipedia article on the collision between Kosmos-2251 and Iridium 33 (https://en.wikipedia.org/wiki/2009_satellite_collision http

According to a source for the Wikipedia article on the collision between Kosmos-2251 and Iridium 33 (https://en.wikipedia.org/wiki/2009_satellite_collision https://en.wikipedia.org/wiki/2009_satellite_collision), ~1000 of these would be from Kosmos-2251 (Russia) and ~300 from Iridium 33 (USA). Looking at the graph on that source (http://www.spacenewsmag.com/feature/10-breakups-account-for-13-of-cataloged-debris/ http://www.spacenewsmag.com/feature/10-breakups-account-for-...) there are several more from other failed satellites from the USA and Russia (which makes sense, given the amount of satellites these two countries have launched).