Donald Trump Jr., the president's son, takes a bathroom break after nearly two hours before the Senate Intelligence Committee, as part of the panel’s ongoing investigation of allegations of Russia’s interference in the 2016 U.S. presidential election, on Capitol Hill in Washington, U.S., December 13, 2017. REUTERS/Jonathan Ernst

WASHINGTON (Reuters) - U.S. President Donald Trump’s eldest son met with the U.S. Senate Intelligence Committee on Wednesday as part of the panel’s investigation into Russia, the 2016 U.S. election and whether his father’s election campaign colluded with Moscow.

Donald Trump Jr. arrived at a Senate office building shortly after 10 a.m. Capitol police officers tried to keep journalists from witnessing his arrival, but he was spotted by reporters as he rushed to a room the committee uses for classified briefings.

He testified for nine hours, a person familiar with the matter said.

U.S. intelligence agencies said after Trump’s victory in the November 2016 presidential election that they had concluded Russia sought to influence the campaign to boost Trump’s chances of defeating former Secretary of State Hillary Clinton, his Democratic challenger.

Moscow has denied any such activity, and Trump has dismissed talk of possible collusion as a “witch hunt” led by Democrats disappointed about his victory.

The Senate committee is conducting one of the main congressional investigations. Richard Burr, the panel’s chairman, told reporters on Tuesday he expected its probe to last into 2018, but likely not for many months into the new year.

Department of Justice Special Counsel Robert Mueller is also investigating the matter.

Trump Jr. testified to the House Intelligence Committee last week.

Lawmakers are interested in talking to him about a meeting with a Russian lawyer in June 2016 at Trump Tower in New York at which he said he hoped to get information about the “fitness, character and qualifications” of Clinton.