WASHINGTON (Reuters) - Two former U.S. officials, intelligence director James Clapper and deputy Attorney General Sally Yates, will testify next month in a congressional investigation into alleged Russian meddling in the 2016 U.S. presidential election, the Senate Judiciary Committee said on Tuesday.

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Congressional committees began investigating the issue after U.S. intelligence agencies concluded Russian President Vladimir Putin had ordered hacking of the Democratic political groups to try to sway the election toward Republican Donald Trump. Moscow has denied any such meddling.

Clapper, the former director of national intelligence, and Yates, the former deputy attorney general, will testify on May 8 before the Judiciary Committee’s subcommittee on crime and terrorism, Judiciary Chairman Charles Grassley said in a statement.

More witnesses may be added.

Senate Democratic leader Chuck Schumer said on Tuesday he was “a little troubled” that the Senate Intelligence investigation appeared to be moving slowly.

Reuters reported on Monday that the probe, the Senate’s main investigation into allegations of Russian election meddling, has a much smaller staff than previous high-profile congressional investigations.

Clapper, Yates and another official who also served under former President Barack Obama, ex-CIA Director James Brennan, were 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, recused himself from the Russia investigation in April after receiving information about intelligence agency surveillance of foreign nationals that swept up some information about members of Trump’s transition team.

His decision to hold a news conference and discuss the information with Trump before disclosing it to Democrats raised questions about whether he could lead a credible investigation.

The House committee on Friday invited Yates, Clapper and Brennan to appear at a public hearing to be scheduled after May 2.

The Judiciary subcommittee’s chairman, Republican Lindsay Graham, said he wanted to ask Clapper and Yates whether they knew about a court order allowing FBI surveillance of the communications of Trump campaign adviser Carter Page in 2016.

The Washington Post reported this month that such a warrant had been issued. Clapper said in a March television interview he was not aware of any wiretapping of Trump or advisers.

“I want to get to the bottom of this,” Graham said on Fox News.