Two plants that come to mind are:
Adenophora(Ladybells)
Campanula rapunculoides(Creeping/Rampion Bellflower)
I've not personally encountered either, so I don't know if the leaves or roots match, but if you do a search on them maybe you will figure it out.
Remy

Thanks remy, I have creeping/rampion bellflower. "Highly invasive" the sites I read tell me - they're not kidding (sigh)

Creeping Bellflower is the bane of my existance. A previous homeowner planted it !! It has now spread and taken over every area of the yard. The roots go very deep and cannot be pulled by hand. The plants have small roots that will break away from the thick tuberous roots that live very deep. I've tried digging it out in several spots, but even the smallest fragment of root left behind will regrow. It crowds out everything. It has moved through all the flower beds, the lawn, under sidewalks to other beds. This yard is a lost cause because of that plant.