Comey and Admiral Mike Rogers, the director of the National Security Agency, made clear that their investigation of Moscow and November's U.S. elections could last for months.
Appearing before a congressional panel, Comey also publicly challenged Trump's claim that former President Barack Obama wiretapped his 2016 campaign headquarters in Manhattan's Trump Tower.
The two officials spent 5-1/2 hours before the House of Representatives Intelligence Committee in testimony marked by starkly partisan divides between the panel's majority Republicans and Democrats.
Comey refused to back away from his claim that Russian President Vladimir Putin did not simply want Democratic presidential candidate Hillary Clinton to lose the election; he wanted Donald Trump to win.
Trump created a controversy in early March when he tweeted without giving evidence that Obama had wiretapped his campaign while the businessman competed against Clinton.
"With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets," Comey said.
The committee is one of several in the U.S.Read more at Reuters