US President Donald Trump, who has repeatedly called the investigation a "witch hunt", has criticised Mueller for agreeing to testify before Congress after concluding the Russia probe, saying the testimony will not yield any new information.
The former US special counsel concluded his report on the Russian investigation in April. The report confirmed that the probe had found no evidence of collusion between the Trump campaign and Russia in the run-up to the 2016 US presidential election. However, Mueller reported about 10 episodes that might constitute obstruction of justice by the US president.
Russia has denied any claims of interference in the US political system.
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