A K1 visa, also known as a fiance visa, is a card that will permit someone intending to marry an American citizen to move to the United States, and eventually to obtain a permanent green card and live as a legal resident in the United States.
To obtain a K1 visa, the American citizen must sponsor the immigrant to come to the U.S. by filling out a form I-129F, which is mailed to Immigration Services along with detailed information about both applicants. This information includes proof of prior relationship, lack of criminal record, and proof that both parties are not currently married.
After the initial application is approved, the immigrant must undergo a complete medical examination in his native country, and then attend an interview session with the US Embassy staff in his country. The interviewer may ask questions about the couple's relationship, and look at documentation including photos, emails, and letters between the couple and their family members. If the applicant is approved, he will be approved for the K1 visa, and can travel to the US immediately.
Once the K1 visa has been approved, there is a strict timeline that the applicants must follow. The nonresident has a period of six months to enter the United States before the K1 visa expires. Once she has moved, the couple has 90 days from time of entry to get married. If the immigrant fiance has children that will be coming to the United States, they will be issued K2 visas, instead of the K1 visa.
After the couple has gotten married, the nonresident can file for an "adjustment of status," which will change his K1 visa to a conditional permanent resident, and he will be issued a temporary green card. At this point, the couple and any children will attend a more thorough interview with Immigration officers, in which personal questions may be asked to ascertain whether the relationship is valid. If the officer has serious doubts about whether the couple is legitimate, the immigrant may risk losing his permanent resident status, and will have to return to his home country.
However, if all goes well, the immigrant will be verified as a legitimate spouse, and will be permitted to work and live freely in the United States. Three years after the filing of the temporary green card, she will be eligible to apply for US citizenship, if she so chooses. All in all, it can take as long as 5 or 6 years from the time of applying for a K1 visa to eligibility for American citizenship.