And so they call Har-Shalom, who runs a nonprofit detective agency that specializes in sniffing out long-lost Jewish ancestry. His agency, called Shorashim (Hebrew for "roots"), is funded in part by the Israeli government. Each year he takes on roughly 1200 cases that test his fluency in Yiddish and Russian dialects, his familiarity with czarist and Soviet history, and his patience for combing through old Soviet archives. He then presents his findings to a rabbinic court, which almost always accepts his expert opinion about a citizen's Jewish identity.

Across thousands of years of Jewish history, seldom did a person need to prove to be a member of the tribe. The Shulchan Aruch, the authoritative 16th-century summary of Jewish religious code, states that for purposes of marriage, anyone claiming to be Jewish can be trusted. Things got complicated when the Iron Curtain fell and hundreds of thousands of Soviets bolted to Israel, where they were welcomed under a long-standing law granting citizenship to anyone with at least one Jewish grandparent (non-Jewish spouses and children were also welcomed). The rationale: if having one Jewish grandparent was enough to brand you Jewish under Nazi race laws, it was enough to grant you refuge in the Jewish state.

But many rabbis questioned the newcomers' lineage. According to the Orthodox definition, several hundred thousand of them were not Jewish. If they intermarried with Israeli Jews, the rabbis feared, the very existence of the Jewish people could be in danger.

To set matters straight, Israel's rabbinate created a new procedure, vaguely called "clarification of Judaism." Every immigrant applying for a marriage license would have to prove Jewish lineage going back at least two generations, and sometimes many more. The government rabbis never established clear rules for conducting these checks. They decided, though, that beyond documentation classifying a person as Jewish, he or she should know Jewish language and traditions, and possess a typical Jewish name.

These guidelines proved problematic for many Soviet immigrants who had little knowledge of Jewish customs -- in the Soviet Union, synagogues had been turned into puppet theaters and gymnasiums -- or had Russified their Jewish-sounding names to avoid discrimination. The classification of Jewish nationality inscribed on their Soviet-era documents, once a hindrance in obtaining jobs and university admission, became the only remaining hope to prove their pedigree. But here, too, was a snag: Soviet emigrants had often been prohibited from taking many of their vital documents out of the country, forced to surrender their original birth certificates in exchange for official government copies. In Israel today, those official government copies are worth little. In the late 1980s and early 90s, plenty of non-Jews bribed Soviet clerks to issue them new documents listing them as Jewish so they could emigrate to Israel. As a result, Israeli officials no longer trust copies of Soviet documents, only originals -- many of which are locked away in archives closed to the public. For prying Israeli detectives, those repositories are a treasure trove.