When does a person from another country becomes a citizen of the United States?

OK, my dad got a jury duty (state of California). He's got all the requirements but we're not sure about him being a citizen of the US. He is from Europe, has a working VISA, lived here for over 2 years, has a social security number and speaks english on a proficient level. Can someone tell me and hopefully give me a website where it states the requirments of a foreign person becoming a citizen? You'll be a really good help!


Answer:
Unless he took the Oath of Citizenship and received his Naturalization Card, he's not a citizen yet. (He hasn't lived here long enough to apply, so he can't have completed that yet.)
Juries can inclued resident aliens...so you don't need to be a citizen to be on a jury.