Is this a portobello?

First off I would like to know if this is a portobello? I am not much on identifying as yet still mostly just reading about culturing and mushroom life cycles etc.

I think it is one, if it is there is a funny story here about how nature will find a way.

At my work we have a company BBQ once a year for all the employees and vendors and all their families. Every year we make mushrooms in one form or another. Generally portobellos and shiitakes. I found these growing very near where we sometimes clean and wash them and definitely cook and serve them. So if these are portobellos they have naturalized most likely from spores but possibly from stems or bits and pieces that came loose from one or the other actions listed above.

Thanks,

Jeff

a few more pics

few more

last ones

yes, ist one of agaricus species for sure, looks like agaricus augustus to me but this doesnt matter they are all similar and edible except one - agaricus xanthoderma, which you can distinguish if you cut the stem just above ground - if it becomes yellow and smells like chemicals you can be shure its this. i poisoned myself once with it but it was nothing much, just vomiting and pain in stomach for hour or two. but be careful.