President Trump listens to questions from the media as he meets with Colombian President Juan Manuel Santos in the Oval Office on May 18, 2017. (Photo by Kevin Lamarque/Reuters)

President Trump has retained the services of a trusted lawyer, Marc E. Kasowitz, to help him navigate the investigations into his campaign and suspected Russian interference in last year’s election, according to people familiar with the decision.

Kasowitz, who has known Trump for decades, has represented Trump in numerous cases, including on his divorce records, real estate transactions and allegations of fraud at Trump University. He is a partner at Kasowitz, Benson, Torres & Friedman in New York.

With the appointment last week of a special counsel to probe alleged Russian meddling in the election and possible collusion with the Trump campaign, the stakes have been raised considerably for the Republican president and his associates. Trump has repeatedly denied that he did anything improper and has said that he has been told he is not under investigation.

The White House had no immediate comment on Kasowitz’s hiring. Those who confirmed the decision spoke on the condition of anonymity because they were not authorized to discuss the move.

In recent days, Trump has been looking at pulling together a unit of lawyers outside the White House to guide him as he responds to the ongoing federal probe and to congressional investigations. It was not immediately clear Tuesday night what other lawyers might be joining Kasowitz as part of such a team.

According to several people familiar with the deliberations, other attorneys under consideration have included Robert J. Giuffra Jr.; Reid H. Weingarten; and Theodore B. Olson.

Former CIA director John Brennan testified May 23 before the House Permanent Select Committee on Intelligence about Russia's influence on the 2016 presidential election. (The Washington Post)

Even as Trump travels abroad this week, the Russian probe has continued to make headlines.

On Tuesday, former CIA director John Brennan told the House Intelligence Committee that he had alerted the FBI to a troubling pattern of contacts between Russian officials and associates of the Trump campaign last year, shedding new light on the origin of a criminal probe that now reaches into the White House.

The outside legal team would be separate from the White House Counsel’s Office, which is led by Donald F. McGahn, who served as the Trump campaign’s attorney. In past administrations, presidents including Bill Clinton have named outside counsel to help them navigate thorny legal problems.

A potential complication for Kasowitz is that former senator Joseph I. Lieberman, among Trump’s leading candidates to head the FBI, is currently a senior counsel at his firm.

Were Lieberman officially chosen to run the FBI, and Kasowitz chosen to help with Trump’s legal advice, both men — the one leading the organization investigating possible Russian collusion and the one offering Trump legal counsel on that very issue — would hail from the same firm, likely presenting a conflict of interest.

The White House did not respond this week to requests for comment about how Trump would pay for his outside legal team, the cost of which cannot be covered by the federal government. But campaign finance lawyers said Trump could probably draw funds from his reelection committee to cover legal expenses related to the Russia inquiries, including money donated this year.

Among those also under consideration to join the team, Giuffra, Olson and Weingarten have already spoken with senior administration officials about the team, said a person familiar with the process.

Giuffra, a partner at Sullivan & Cromwell in New York, is the coordinating counsel for Volkswagen.

Olson, a former U.S. solicitor general, rose to prominence in 2000 when he argued the Supreme Court election case that delivered the electoral victory to George W. Bush. He later teamed up with his former Democratic adversary in Bush v. Gore, David Boies, to successfully overturn the 2008 California ballot measure outlawing same-sex marriage.

Olson’s then-wife Barbara was killed in the Sept. 11, 2001, terrorist attacks when the plane on which she was a passenger was flown into the Pentagon. Olson, a partner at Gibson, Dunn & Crutcher’s Washington office, is now married to Lady Booth Olson, a self-proclaimed lifelong Democrat.

Weingarten, a high-powered attorney at Steptoe & Johnson in Washington known for his folksy style, is a somewhat unlikely choice because he has represented Democratic clients and is close friends with Eric H. Holder Jr., who served as attorney general under President Barack Obama. Holder and Weingarten met during their early years at the Justice Department.

Robert Costa contributed to this report.