WASHINGTON - Rod Rosenstein, the US official overseeing an investigation of ties between President Donald Trump's election team and Russia, warned a group of conservative Republicans on Tuesday that his department would not be "extorted" and criticized them for leaking a draft resolution calling for his impeachment.Rosenstein, US Deputy Attorney General, has been under fire since last May when he appointed Special Counsel Robert Mueller to investigate Russia's suspected interference in the 2016 election and possible collusion by the Trump campaign. The so-called House Freedom Caucus this week leaked a draft resolution that said Rosenstein should be impeached for not promptly responding to their requests for documents on FBI investigations.Rosenstein told an event in Washington on Tuesday: "I just don't have anything to say about documents like that that nobody has the courage to put their name on and that they leak in that way.""I can tell you that there are people who have been making threats privately and publicly against me for quite some time and I think they should understand by now the Department of Justice is not going to be extorted."The Kremlin denies allegations made by US intelligence agencies that it interfered in the election to try to help Republican candidate Trump win it.Trump denies any coordination with Moscow officials by his campaign team, and he has repeatedly said the investigation is a political witch hunt.A number of Republican-led committees in the US House of Representatives have opened inquiries into the Federal Bureau of Investigation's handling of several matters.They include an investigation of 2016 Democratic election candidate Hillary Clinton's use of a private email server while she was US Secretary of State and whether the Justice Department made missteps when it applied to the United States Foreign Intelligence Surveillance Court (FISA) for a warrant to conduct surveillance on Carter Page, a former Trump campaign aide.Republican lawmakers have repeatedly demanded access to records on those and other subjects. The Justice Department has said it is working to meet the requests while maintaining the integrity of open investigations."Everybody in the Department takes an oath," Rosenstein said on Tuesday. "And if they violate it, they are going to be held accountable."The House lawmakers who drafted the articles of impeachment, he added, "know that I'm not going to violate mine."