The probe being led by Senator Graham and Senator Whitehouse is one of three main congressional investigations of Russia and the 2016 US election. The FBI and US intelligence agencies are conducting separate investigations.
Director Clapper, Deputy Attorney General Yates and another official who served under President Obama, former CIA Director John Brennan, had been scheduled to testify to the House of Representatives intelligence committee in March, but that hearing was canceled by the panel's chairman, Republican Devin Nunes.
Nunes, a Trump ally, has since recused himself from the Russia investigation amid concerns that he was too close to the White House to lead a credible probe.
Deputy Attorney General Yates, Director Clapper and Director Brennan are now due to appear at a public hearing of the House committee that has not been scheduled.
Source : http://www.express.co.uk/news/world/801621/Donald-Trump-Barack-Obama-Russian-meddling-US-election