(CNN) President Donald Trump is expected to hire attorney Marc Kasowitz to represent him on matters related to the inquiry into his campaign's alleged ties to Russia, according to a senior administration official and a person close to Trump.

Kasowitz has represented Trump on a "wide range of litigation matters for over 15 years," according to the biography posted on the website for his firm, Kasowitz Benson Torres LLP.

Kasowitz has not responded to requests for comment.

The President's legal defense has become increasingly complicated as more details have surfaced in the investigations into Russia's meddling in the 2016 election. The decision to retain Kasowitz comes as there are multiple congressional probes into the Trump campaign's contact with Russians, as well as an FBI investigation.

White House lawyers have even been researching impeachment procedures in order to prepare for such a possibility, CNN reported last week . Close advisers to Trump, including lawyer Michael Cohen, visited the White House last week to discuss the potential need to hire a personal legal team to represent the President.

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