The move was earlier reported by The Associated Press, citing a person familiar with Sterling's legal strategy.
Investigators were given a six-figure budget over the next 30 days to examine the league's finances, allegations of previous discriminatory conduct and compensation to past commissioner David Stern and current commissioner Adam Silver, said the AP source, who spoke Thursday night on condition of anonymity. The person wasn't authorized to talk publicly.
The person said the investigators also are looking into whether other owners made any off-color jokes, or racist or sexist remarks.
"The gloves are off, as they say," the person said. "Have them dig up all the dirt they can find."
Sterling, 80, is suing the NBA for $1 billion in federal court after the league tried to oust him as Clippers owner for making racist remarks to a girlfriend that were recorded and publicized. Silver fined him $2.5 million and banned him for life.
The suit alleges the league violated Sterling's constitutional rights by relying on information from an "illegal" recording. It also says the league committed a breach of contract by fining Sterling and that it violated antitrust laws by trying to force a sale.
"Now his objective is to demonstrate for everybody that the NBA is a damned hypocrite," Sterling attorney Bobby Samini told ESPN.com on Wednesday. "We're going to pull every case against the NBA. Then we'll demonstrate that the culture of racism and gender discrimination is born at the NBA, where Adam Silver has worked in a high-level position for a long time."
In addition, Sterling's attorneys will face off with his estranged wife's attorneys in probate court during a four-day hearing scheduled for July.
The probate court hearing centers on whether Shelly Sterling had the right to unilaterally negotiate a $2 billion deal to sell the Clippers to former Microsoft CEO Steve Ballmer. Shelly Sterling's attorney, Pierce O'Donnell, said she made the deal as the sole administrator of the Sterling Family Trust, which owns the team, after two doctors determined her husband was mentally "incapacitated." Donald Sterling is fighting that conclusion and her authority to sell.
The person who spoke to the AP said Donald Sterling reluctantly agreed to hire private investigators after this week's legal proceedings in probate court. The NBA submitted a legal filing Wednesday urging a judge to confirm Shelly Sterling's authority to sell the team.
"He realized these guys will literally go to any low to get this sold," the person said. "Even if it gets [him] nothing other than exposing all these guys and shaking up the league and seeing a change in the leadership of the league, it'll be worth it to him."
The firms will be engaged in an "audition" of sorts, the individual said: "Whoever delivers the best [material] is going to stay on for the next round, and it might be all four of them ... 29 owners is not a small undertaking."
ESPN.com's Ramona Shelburne and The Associated Press contributed to this report.