"I never worked for Russia, and you know that answer better than anybody," Trump said on the South Lawn.
"It's a whole big fat hoax. It's just a hoax," Trump said of the FBI's Russia investigation.
Trump was responding to a report in The New York Times about the FBI opening a counterintelligence investigation looking into whether Trump was working for the Russians.
When he was asked by a friendly Fox News interviewer on Saturday whether he was working for Russia, Trump offered a non-denial, saying only he was insulted by the question.
On Monday, he was more definitive and criticized FBI officials for what he said was anti-Trump bias.
"The people doing that investigation were people that have been caught that are known scoundrels," he said. "I guess you could say they're dirty cops."
He used familiar attack lines against Lisa Page, Peter Strzok, Andrew McCabe and James Comey, and said the FBI had experienced a turnaround after those officials' departures.