Robert Mueller, who has been appointed special counsel to investigate possible collusion between President Donald Trump's 2016 campaign team and Russia, is a former FBI director who took over the bureau just one week before the 9/11 attacks on the US.

While the 72-year-old’s tenure went on to be defined by the country’s counter-terrorism efforts, the former federal prosecutor’s career is also marked by high-profile investigations, including the Lockerbie bombing, and a standoff with the Bush administration over domestic wire-tapping.

The appointment by the US Justice Department followed a week of turmoil for the White House amid rising demands for an independent probe of alleged Russian efforts to sway the outcome of November's presidential election in favour of Mr Trump and against Democrat Hillary Clinton.

Pressure has been building on Mr Trump since his firing of James Comey, head of the Federal Bureau of Investigation, who had been leading a federal probe into the matter.

Career as a prosecutor

After serving as a Marine officer in the Vietnam War, for which he was awarded a Bronze Star, Purple Heart and two Navy Commendation Medals, Mr Mueller worked for 12 years in the US Attorney offices.

Known for his tough, no-nonsense managerial style, he investigated and prosecuted crimes ranging from major financial fraud to public corruption cases.