WASHINGTON (Reuters) - Members of the U.S. House Intelligence Committee on Thursday interviewed the head of a data analysis firm to determine whether Donald Trump’s election campaign team sought his help to find thousands of emails missing from Hillary Clinton’s private server, three sources familiar with the session said. Trump’s campaign hired Alexander Nix and the company, Cambridge Analytica, in June 2016 and paid it more than $6.2 million through last December, according to Federal Election Commission records. The month after his firm was hired, Nix emailed WikiLeaks founder Julian Assange for help tracking down some 33,000 emails that Clinton supposedly had deleted from her private server, the Daily Beast and The Wall Street Journal reported in October. Nix wanted to convert the missing and potentially damaging emails into a searchable database for use by the Trump campaign or a pro-Trump political action committee, the Journal reported.  WikiLeaks confirmed to Reuters on Thursday that it was approached by Nix, but a representative said it turned down his request.     The intelligence committee is investigating possible collusion between Trump’s campaign team and Russia during the 2016 election. The matter is also being investigated by Special Counsel Robert Mueller and three other congressional committees. Russia has denied meddling in the election, and Trump has said there was no collusion between Moscow and his campaign.   Representatives Elijah Cummings and Jerrold Nadler, the top Democrats on the House Oversight and Judiciary committees, on Thursday asked their Republican committee chairmen to issue subpoenas for documents from Cambridge Analytica and Giles-Parscale, another data analysis firm that worked for the Trump campaign. Spokesmen for Cambridge Analytica and Giles-Parscale did not immediately respond to emails requesting comment.          Cummings and Nadler said they sent their letter after the two companies declined to answer questions about whether they had contacts with foreign actors during the 2016 presidential campaign. The sources said the intelligence committee members met at a Washington law office on Thursday to conduct the interview with Nix by Skype because he was not in the United States. The sources said the interview lasted some 90 minutes. The CIA, Federal Bureau of Investigation and the National Security Agency said in January that Russia used propaganda, social media and other means to meddle in the 2016 election to try to help Trump defeat Clinton.         In their report, the agencies said Russian intelligence services hacked emails and other documents from the Democratic National Committee and other Democratic Party organizations, and used Wikileaks to release them.  