A Philadelphia historian sparked a days-long — and so far fruitless — archival search when she challenged her blog readers to take an “impossible” test purportedly once given to prospective black voters in Louisiana.

The test, which asks the taker to “spell backwards, forwards” among other tasks, went viral on the Internet after it posted on a noted civil rights history website. The Tennessee State Archives put a copy in its collections. Teachers are using it in their history lessons. However, history experts in Louisiana do not have a copy of it.

“I suspected that was a hoax,” Andrew Salinas, reference archivist for the Amistad Research Center at Tulane University, said Wednesday.

A former civil rights worker who insists the test was given offered another explanation. Jeff Schwartz said Louisiana might have been reluctant to preserve an embarrassing chapter in its history....