When the man appeared before a federal judge in Manhattan to be sentenced in a drug case, he had a lawyer by his side, supporters in the courtroom and letters attesting to his character. Only one thing was missing: his true identity.

Throughout his trial and conviction, the defendant had claimed to be someone he was not, and no one had any idea who he really was.

“I sentence people almost every day,” the judge, Richard J. Sullivan, said, “and I will tell you candidly, I am not aware of anybody who has done what you have done in this case.”

Court records had listed the man as “Fnu Lnu,” shorthand for “First name unknown, Last name unknown.” The acronym is often used in the early stages of a criminal case, when investigators cannot identify a voice on a wiretap, or the identity of someone picked up in an immigration sweep.