Key regulatory authorities in the US enforcing antitrust laws

Which are the key regulatory authorities in the US enforcing antitrust laws and thus scrutinize (and then approve or challenge)
acquisitions of US-based publicly traded companies by other US firms or by foreign firms? What ownership threshold (i.e.
percentage number of purchased shares of a publicly traded company by another domestic firm or domestic investor) needs to be
crossed in the US to constitute an acquisition, and what is the percentage number for cross-border investments to constitute an
acquisition?