Vice President Mike Pence speaks during a conference on Prosperity.
The obstruction of justice investigation into Trump began days after Comey was sacked on May 9, according to people familiar with the matter, the Washington Post said. Cullen also previously served as Virginia's Attorney General in the late 1990s. Richard Cullen has a long record as a high-profile attorney in Washington, particularly in Republican circles. Paul Trible during the Iran-Contra investigation and was a member of former President George W. Bush's legal team during the 2000 Florida recount, according to his official biography.
Cullen served as a USA attorney under President George H.W. Bush and is now the chairman of the McGuireWoods LLP law firm in Richmond, Virginia, according to ABC News. The law firm referred questions back to the vice president's office.More news: Jennifer Lawrence's plane makes emergency landing after both engines fail
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Pence's announcement comes one day after the Washington Post reported that Mueller is investigating President Donald Trump for possible obstruction of justice - an investigation triggered by Trump's shock decision to fire former FBI director James Comey.
Mueller is investigating alleged Russian interference in the 2016 US presidential election and possible collusion with the Trump campaign. As head of the Trump campaign's transition committee, he was involved in the hiring of Michael Flynn as national security adviser.
But obstruction came into play after Trump fired Federal Bureau of Investigation director James Comey last month.