When J. Bryan Quesenberry first learned that the federal government was sending out hundreds of billions of dollars to help businesses survive during the Covid-19 pandemic, he thought: “There’s going to be fraud here. There just has to be.”

A few months later, Mr. Quesenberry started sifting through a list of businesses that received Paycheck Protection Program loans, which were intended to help small businesses ravaged by the pandemic continue paying their employees. The Oregon lawyer said he knew businesses were not allowed to receive more than one loan during a single round, so he searched for “double dippers.”

He soon found dozens of businesses across the country that appeared to improperly obtain P.P.P. loans. During the summer of 2020, Mr. Quesenberry started suing those firms to try to help the government recover funds.

“It just blows my mind,” Mr. Quesenberry said. “That’s tax money that comes out of your pocket and that comes out of my pocket.”

As federal officials try to retrieve billions in stolen pandemic relief funds, private citizens are scouring public data, company websites and social media pages to help identify potential cases. Those who have filed suits say they are motivated by the desire to root out wrongdoers and expose corporate fraud.

But there is also a strong financial incentive. Under the False Claims Act, private citizens can file lawsuits on behalf of the federal government against those who may have defrauded the United States. If the government recovers funds, those citizens can typically earn between 15 and 30 percent of that amount.