The lawyer privately advising Donald Trump on the investigation into Russia’s interference in the 2016 election is head of a law firm that was involved in the sale of a prestigious piece of New York real estate to Jared Kushner, the US president’s son-in-law, in a deal that could fall under the spotlight of the same inquiry.

Marc Kasowitz, a member of the New York bar who has represented Trump in his business dealings for 15 years, was brought on board by the president last month to provide personal legal advice relating to the Russian inquiry now being conducted by special counsel Robert Mueller. The appointment has placed Kasowitz at the center of the legal maelstrom over the investigation into potential collusion between Russia and elements of Trump’s presidential campaign.

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An investigation by the Guardian has found that Kasowitz’s law firm, Kasowitz Benson Torres, legally represented the owners of the former New York Times building in Times Square, Manhattan, in a 2015 deal in which part of the property was sold to Kushner for $296m.

The Washington Post has reported that a subsequent loan of $285m from Deutsche Bank to Kushner Companies, relating to the purchase of the building, could fall under the remit of the Mueller investigation given Deutsche Bank’s scandal-riven reputation. The involvement of Kasowitz’s firm as a key legal player in the initial sale adds a further possible twist as the special counsel’s inquiry gathers momentum.

Questions have already been raised about possible conflicts of interest between the lawyer’s role as Trump’s private attorney in the Russian inquiry and his work for various other clients, among them Russia’s largest bank OJSC Sberbank, which he represents in a corporate dispute lodged in US federal court.

The Guardian asked Kasowitz, via his spokesman, to respond to the potential conflict of interest relating to his firm’s role as attorney on the sale of the Times Square building to Kushner but he did not respond before publication. After publication, the spokesman send in a statement: “There are no conflicts under any standard or by any definition.”

Facebook Twitter Pinterest The old New York Times building at 229 W 43rd St. The building is 51% owned by Jared Kushner. Photograph: Simon Leigh/The Guardian

Trump’s connections with Kasowitz’s law firm go much further than just his personal attorney, raising other potential conflict of interest issues. Another of its partners, David Friedman, was appointed by the president as ambassador to Israel; its senior counsel, Joe Lieberman, was considered by Trump as replacement director of the FBI after the president fired James Comey but pulled out of the running, citing potential conflicts of interest with Kasowitz as the president’s private legal counsel; and yet another partner, Edward McNally, is reportedly in the running to replace Preet Bharara as US attorney for the southern district of New York, following a similar Trump sacking.

ProPublica has alleged that Kasowitz himself bragged to friends that he played a role in having Bharara fired, by telling Trump: “This guy is going to get you.” One of the major investigations conducted by the southern district of New York under Bharara was allegedly to look into Deutsche Bank’s involvement in alleged Russian money-laundering.

The Guardian invited Kasowitz to confirm or deny the ProPublica account, but he did not respond.

The old New York Times building is a neo-gothic styled building fronting Times Square, which had been a home to printing presses since the first year of Woodrow Wilson’s presidency. Located at 229 West 43rd Street in Manhattan, the building was declared a New York City designated landmark in 2000.

At the time of the sale to Kushner, the building was owned by Lev Leviev, an Israeli citizen born in the former Soviet Union in what is now Uzbekistan. His company, Africa Israel Investments, bought the Times Square property in 2007 for $525m.

State records show that Africa Israel’s Delaware LLCs, including its subsidiary that bought the former New York Times building, are registered at 40 Wall Street, Trump’s property over the road from the New York Stock Exchange. The office block has “The Trump Building” emblazoned over its entrance in gold capital letters, and was also the home of the now defunct Trump University.

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Leviev is one of Israel’s richest businessmen, having made a fortune partly in diamond mining in Africa. He has claimed that he is a “true friend” of the Russian president, Vladimir Putin.

In a statement to the Guardian, the Leviev Group of Companies said that Leviev had indeed met Putin a few times though only in his capacity as president of the Federation of Jewish Communities of the Commonwealth of Independent States (the former Soviet Union). Leviev “does not have a personal relationship with the Russian premier” and the comment that he was a “true friend” referred to Putin’s help to the “Jewish community in Russia”.

Leviev’s company has had a number of contacts with the Trump family circle that go further than Kushner. Rich Marin, a former chairman and CEO of Africa Israel USA, has told the Guardian that he had a meeting with Trump himself as well as his daughter Ivanka about the possibility of creating a Trump hotel inside the Times Square property.

That deal never materialized, and it was several years later that Ivanka’s husband, Jared Kushner, stepped into the breach. Filings with the Securities and Exchange Commission (SEC) show that the sale of the retail portion of 229 West 43rd Street to Kushner was made in an off-market transaction of the sort normally used by owners desiring a quick sale, where speed is more important than price.

Given the lack of competition inherent in such trades, they often give an advantage to the purchaser seeking to acquire property at a bargain.

Kushner acquired the building for $296m from Africa Israel USA and its partner in the deal Five Mile Capital, with Kasowitz’s law firm representing the sellers. The transaction included four storeys of retail space and two sub-basement floors.

Facebook Twitter Pinterest The old Times building pictured in 1924. Photograph: New York Times/Redux/Eyevine

The upper 12 floors of the old New York Times building, which are used for offices, were sold to Blackstone in 2011 for $160m. When both sales are put together, Leviev let go of the entire building after owning it for eight years and committing millions of dollars in renovations for a total price that was beneath the $525m he had originally paid for it.

The Guardian contacted Kushner Companies and Five Mile Capital about the sale, but neither commented.

In a statement to the Guardian, the Leviev Group of Companies said: “The Kushner Companies’ offer for the retail space was the most attractive offer ever submitted, and was higher than the building’s appraisal.”

Kushner sealed the refinancing deal with Deutsche Bank and SL Green over his half of the former New York Times building last October, in the dying days of the presidential campaign. The package, reported by Kushner’s own news magazine, Commercial Observer, amounted to $370m – $74m more than Kushner had paid for it.

The president of Kushner Companies, Laurent Morali, told the Washington Post that the discrepancy in price was a result of the “dramatic turnaround” that they had effected in filling up vacant rental units with high-profile outlets. “We had a vision for the property when we purchased it that no one else had, and are proud to say that we executed on it,” he said.

Deutsche Bank is the biggest lender to Trump, having provided $364m in loans. The German bank has been hit by a series of scandals including Russian money laundering, and was ordered earlier this year to pay more than $600m in fines for failing to prevent the secret and improper transfer of more than $10bn out of Russia.

The Guardian revealed in February that Deutsche Bank had itself conducted an investigation of Trump’s personal account to see whether there were any suspicious links to Russian entities.

Senior Democrats in Congress have raised concerns about possible conflicts of interest relating to the Trump administration and the president’s own financial debts to Deutsche Bank. In March, a group of Democrats on the House financial services committee wrote a joint letter saying that such links “raise serious concerns about whether the president and his inner circle will direct the Department [of Justice] to steer clear of issues that could implicate those who benefited from Deutsche Bank’s trading scheme.”

On Tuesday the Guardian reported that a different Trump lawyer, Jay Sekulow, had approved plans to push poor and jobless people to donate money to his Christian nonprofit, which since 2000 has steered more than $60m to Sekulow, his family and their businesses.

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