US Attorney General Jeff Sessions says he never had conversations with former FBI Director James Comey about his job performance before Mr Comey's firing.
Mr Sessions recommended the firing last month, raising questions about whether he violated his recusal from the investigation into Trump campaign ties to Russia.
But on Tuesday (local time) Mr Sessions testified he had concerns about Mr Comey's job performance even before he was confirmed.
He said he and Deputy Attorney-General Rod Rosenstein discussed it and "we both agreed that a fresh start at the FBI was probably the best thing".
Mr Sessions said he was involved in Mr Comey's firing because he oversees the FBI, adding that "to suggest that a recusal from a single specific investigation" would render him unable to manage the leadership of the FBI would be "absurd".
Mr Sessions' comments come during his testimony to the Senate intelligence committee, which is investigating Russia's alleged meddling in the 2016 presidential election and possible links with US President Donald Trump's campaign.
Earlier Mr Sessions contradicted Mr Comey's testimony about his concerns over a meeting he had with Mr Trump.
Mr Comey testified last week that Mr Sessions did not respond when he complained that he did not want to be left alone with Mr Trump again.
This was after a February meeting in which Mr Comey said Mr Trump told Mr Sessions and others to leave the room before asking him to drop a probe into National Security Adviser Michael Flynn's contacts with Russia.
Mr Sessions testified he was not silent, saying he stressed to Mr Comey the need to be careful about following appropriate policies.
The committee is also asking him whether he met privately with Russian Ambassador Sergey Kislyak at an April 2016 foreign policy event at the Mayflower Hotel in Washington.
Mr Sessions said he was there for a speech by then-candidate Mr Trump and members of Mr Sessions' staff also were there.
He said the suggestion he was "aware of any collusion with the Russian government to hurt this country which I have served for 35 years, or to undermine the integrity of our democratic process, is an appalling and detestable lie".
Mr Sessions added he has no knowledge of "any such conversations by anyone connected to the Trump campaign".
Mr Sessions said he did not have third meeting with the Russian ambassador to the United States.