Trying to remember the title of a book probably from 90s or 2000s. English Paperback.
A Portal opens up in (Florida)? (Pretty sure it was Florida or another SE USA state)
Small dangerous alien creatures start coming through it and attacking people near it.
I 'think' it was just a spontaneity creation (i.e. Not cause by man) But it may have occurred as a result of some experiment that opened it.
Eventually police and SWAT come and start fighting the aliens and kill them all, But then Larger aliens come through and over run the police.
Eventually the National Guard sets up a perimeter, closes in and kill the new aliens. But then even LARGER ferocious aliens come through, and the US Army arrives with tanks, missiles, mortars, etc. and its a stalemate.
I Think the scientist by now have set up detection equipment that can see the radiation coming from or caused by the portal. And they report that they are detection more portals opening up elsewhere in the USA.
Another possible part of the story, some people may have stumbled through the first portal, (or through one of the other ones that are opening up) and the end up in contact with some other intelligent aliens (A different species from the aliens attacking through the first portal) who tell them about the portals. (Whats happening, why they are opening, etc.)