There's at least the case of Anwar al-Awlaki, an American citizen who was killed via drone strike authorized by President Obama. Here's a letter then-AG Eric Holder sent to Congress: https://archive.nytimes.com/www.nytimes.com/interactive/2013...

Some extra details:

Not only Alwaki himself was killed, but his wife, son, daughter, brother in law, nephew, and few other certainly non-involved relatives over a period of many years.

Strikes, and raids made when US was hunting down his family member killed few dozens more people falsely believed to be his relatives.

Obama was very open about killing US citizens so of course it has https://en.wikipedia.org/wiki/Anwar_al-Awlaki is probably the most famous since two weeks after he had him droned we droned his son (also a US citizen).

Trump also killed his daughter.

https://www.nbcnews.com/news/world/seal-american-girl-die-fi...

Extrajudicial killings of American citizens by the US? Yes, several times. It's been a fairly regular event in the middle east.