Help Identify, Giant Fly Excreting Silk? Portland, Oregon US

Hello,

I happened upon this massive black fly in the portland area of Oregon. The width was that of a dime and about twice as long. It also appeared to have a long string of silk attached to its behind. Similar to a spider. I couldn’t tell if it was simply caught up in it or if it was actually excreting it. Based on its rear leg movements it appeared to be coming from its behind. I’ve never seen something like this so I’m just curious as to what it is. Perhaps the experts on here can shed some light. I’m new here, can someone provide guidance on how to posts a picture?

imgur.com/b3FaPll