Kasowitz has represented one of Deripaska's companies for years in a civil lawsuit in New York and was scheduled to argue on the company's behalf May 25, two days after news broke that Trump had hired him, court records show. A different lawyer in Kasowitz's firm showed up in court instead, avoiding a scenario that would have highlighted Kasowitz's extensive work for high-profile Russian clients.
Kasowitz, whose scrappy style in the courtroom mirrors Trump's approach to politics, represented Trump in various matters for more than a decade before he took on either Deripaska's company or Sberbank, according to one of Kasowitz's partners in the firm.
Trump has turned to Kasowitz for matters that include debt restructuring and suing an author who Trump said undercounted his net worth. On Thursday, Kasowitz became the public face of Trump's counterattack on former FBI director James B. Comey, challenging the former federal prosecutor's credibility and calling for Comey to be investigated for leaks after his testimony to Congress.
As Kasowitz takes on his most high-stakes work for Trump yet, the lawyer's Russian clients could cause complications.
"If the behavior of a Russian client of the firm or its relationship with Trump becomes an issue in the investigation, a conflict could arise," said Stephen Gillers of New York University Law School, an expert on legal ethics.
Deripaska has said congressional investigators have contacted his attorneys seeking information about his business dealings with Paul Manafort, a Trump campaign manager during the presidential campaign. More than a decade ago, Deripaska invested in a fund that Manafort set up in the Cayman Islands that bought assets primarily in Ukraine.
The Associated Press reported in March that Manafort "secretly worked for" Deripaska as far back as 2006 to influence politics and business dealings inside the United States to benefit Putin's government. Manafort signed a $10 million annual contract beginning in 2006 and maintained a business relationship until at least 2009, the AP reported.
Deripaska has denied the report, and he sued the AP for libel last month. Deripaska said he "never had any arrangement, whether contractual or otherwise, with Mr. Manafort to advance the interests of the Russian government," according to the lawsuit. In newspaper ads taken out after the AP story, he said, "I want to resolutely deny this malicious assertion and lie."
Former associates of Sberbank, the other Russia-tied Kasowitz client, also have come under scrutiny in media reports.
The bank's former vice president, who is now chief executive of another Russian state-owned financial institution, Vnesheconombank, met with Trump's son-in-law and adviser, Jared Kushner, in December.
Kushner's interactions with the Russian banker are a part of the FBI's investigation into potential coordination between Moscow and the Trump campaign team.
Gillers said Kasowitz's firm should closely monitor potential conflicts. If one arises, the firm probably would have to drop one of its clients, he said.
A White House spokesman did not respond to requests for comment Friday. Michael J. Bowe, a partner at the law firm Kasowitz Benson Torres, declined to say whether the firm had discussed the possibility of potential conflicts arising from its Russian clients. Bowe added that their representation of the Russian firms and Trump "are totally unrelated."
CNN and BuzzFeed previously reported Kasowitz's Russian clients.
Trump hired Kasowitz in 2001 to restructure debt on his firm's Atlantic City casinos. More recently, Kasowitz filed a lawsuit against Timothy O'Brien, arguing that the author of "TrumpNation: The Art of Being the Donald" had libeled Trump by understating the businessman's wealth. Trump lost the case in 2011. O'Brien told The Post last year that Trump used Kasowitz because he "always favored scrappy lawyers and street fighters."
Kasowitz also wrote a letter during the presidential campaign threatening to sue the New York Times for an article that said two women had accused Trump of touching them inappropriately. Kasowitz said at the time it was "nothing more than a politically motivated effort to defeat Mr. Trump's candidacy." No suit has been filed.
On Tuesday, Deripaska's company, Veleron, lost an appeal in federal court in Manhattan in its lawsuit against Morgan Stanley in a complex financial case involving a dispute over a loan on which Veleron defaulted during the height of the Great Recession. Kasowitz was scheduled to deliver oral arguments in the appeal last month. It's not clear whether Kasowitz will continue to represent Veleron.
Records in the case reinforce Deripaska's close ties to Putin. When Deripaska's company ran into financial trouble in 2008 and needed to put up more collateral to cover some its liabilities, Deripaska put in a call to Putin, who authorized the state-run Vnesheconombank, or VEB, to offer his firm a bailout, Deripaska acknowledged in court records.
In 2008, Forbes magazine listed Deripaska as the ninth-richest man in the world. In 2006, the United States revoked his visa to enter the country, citing possible ties to organized crime. He has denied those links, claiming the allegations are part of an effort to smear him.
Kasowitz represents Sberbank in a 2016 lawsuit that is still in its preliminary stages. An owner of a Russian granite-mining business accuses the bank of conspiring with competitors to dismantle his company and seize its assets. The bank has not responded in court filings.
Sberbank was one of the sponsors of the 2013 Miss Universe pageant in Moscow produced by Trump, who owned the competition. The deputy head of the bank at the time was Sergey Gorkov, who met with Kushner in December. Gorkov, a graduate of the academy of the Federal Security Service, or FSB, the domestic successor of the former Soviet KGB intelligence bureau, was named to head VEB in February 2016.
VEB has maintained that Gorkov's meeting with Kushner was part of a new business strategy and was conducted with Kushner in his role as the head of his family's real estate business. The White House has said the meeting was unrelated to business and was one of many diplomatic encounters the soon-to-be presidential adviser was holding ahead of the inauguration.
Rosalind S. Helderman and Tom Hamburger contributed to this report.