Robert Mueller, the former special counsel whose investigation into alleged Russian interference in the 2016 US election defined much of Donald Trump's first term in office, has died aged 81. The cause of his death was not immediately clear. CBS News, the BBC US partner, confirmed his death.

With deep sadness, we are sharing the news that Bob passed away on Friday night, his family told the AP in a statement. His family asks that their privacy be respected.

Mueller previously led the Federal Bureau of Investigation (FBI) from 2001 to 2013, taking the office just days before the 11 September 2001 terror attacks. He is credited with reshaping it into a modern counterterrorism agency.

Mueller is survived by his wife of nearly 60 years, Ann Cabell Standish, their two daughters, and three grandchildren.

Mueller's special counsel inquiry put Donald Trump's 2016 campaign under a microscope, drawing harsh criticism from the US president. Trump wrote on Truth Social on Saturday: I'm glad he's dead. He can no longer hurt innocent people!

Mueller's former employers and colleagues praised him as a longtime public servant. Former Presidents George W. Bush and Barack Obama paid tribute. Bush, who appointed Mueller to lead the FBI, expressed deep sadness over his passing, while Obama called him one of the finest directors in the history of the FBI.

Mueller was noted for his investigation that scrutinized Russia's attempts to influence the election and potential involvement of Trump and his campaign. Despite his work making headlines, Mueller himself rarely spoke publicly. His investigation resulted in multiple indictments and plea deals and made a significant impact on US politics.

Born in 1944, Mueller served in Vietnam and later studied law. He was unanimously confirmed as FBI director in August 2001, serving more than a decade before retiring in 2013.