Russo failed the first two questions and the polygraph operator, Roy Jacob, thought he was a psychopath. Jacob knew he had to tell somebody that things were amiss but he also knew his job at the Jefferson Parish sheriff's office would be at risk.
Here is the internal report of the test sent to Jim Garrison (note that many documents use Jacobs, but his name was Jacob):
The polygraph test was administered on March 8, 1967, well before the preliminary hearing on March 14, 1967. The memo was typed up two months later. Garrison put Russo on the stand despite the report from Roy Jacob.