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. He was 81.

The cause was not immediately known. CBS News, the BBC US partner, confirmed his death.

"With deep sadness, we are sharing the news that Bob passed away" on Friday night, the family said to 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 the FBI into a modern counterterrorism agency.

Mueller's special counsel inquiry put Trump's 2016 campaign under a microscope, drawing harsh criticism from the president.

The president on Saturday wrote on Truth Social: "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.

"A great American died today, one I was lucky enough to learn from and stand beside," wrote former FBI director James Comey, who succeeded Mueller at the agency and whose abrupt firing by Trump led to the Mueller investigation.

In a statement, a spokesman for his former law firm, WilmerHale, called Mueller an "extraordinary leader and public servant and a person of the greatest integrity".

Mueller was born in 1944. After studying politics at Princeton University, he joined the Marines and was sent to Vietnam in 1968.

As a lieutenant, Mueller led a platoon of troops, was wounded twice in battle, and was awarded numerous commendations including the Bronze Star for bravery.

After returning from the war he went to the University of Virginia, where he studied law and graduated in 1973.

In August 2001, Mueller was unanimously confirmed as FBI director by the Senate, and he served at the agency for more than a decade.

Mueller retired from the FBI in 2013, after serving under both a Republican president, George W Bush, and a Democrat, Barack Obama.