KABUL, Afghanistan — If Kabul Bank’s founder, Sherkhan Farnood, had not decided on a summer day three years ago that revenge was worth self-incrimination, he might never have seen the inside of a courtroom.

But the halting and often obstructed investigation he helped fuel that day culminated on Tuesday with the first convictions in the Kabul Bank fraud scandal, a spectacular implosion of corruption that has undermined the credibility of the Afghan government and its Western benefactors.

Mr. Farnood was one of 21 people found guilty on Tuesday. But it was specifically his conviction and that of his chief executive and former bodyguard, Khalilullah Frozi, that American and European officials had warned would be necessary if billions of dollars of international aid was to continue to flow to Afghanistan. The two masterminds were convicted of a crime akin to fraud, sentenced to five years in prison and fined hundreds of millions of dollars, considerably lesser results than prosecutors had sought.

The path to those verdicts had been laid out in dozens of interviews since Kabul Bank nearly collapsed in 2010, with the main players offering details of the fraud scheme and their decision-making, including Mr. Farnood, shareholders and Afghan, American and European officials.