Past cattle mutilations proved to be the work of government pathologists checking the effects of nerve gas releases from Dugway Proving Grounds if I recall correctly.
At least partly not so.
Some whistle blowers have asserted that the critters do about half of them and government personnel do about half of them.
In the beginning, it was all the critters doing.
There are lots of speculations and assertions about what the critters are up to in the project. They range from tissues needed for sustaining their lives to monitoring pollution and radiation effects to tissues needed for cloning humans or critters etc. etc. etc.
We did not have the technology to pull such stunts until the critters gave it to us.