Would the US government be cool with one of its minions pulling off an assasination on US soil?

If a nation that was aligned with the US but not really liked, more like tolerated, found a US citizen to be a heinous criminal of their nation and snuck in by way of Canada assassinated him/her slipped back out of the country, after which they informed the State Department what they had done, how do you feel the US government will feel about it? Would the government feel that US sovereignty had been violated? If they were a valuable lackey but not but expendable because they were not highly strategic would they be attacked? Would their embassy here be closed?