The Prime Minister had previously said terrorist Usman Khan was back on the streets because of laws introduced by a Labour “leftie government”.
But Mr Johnson, after facing criticism for “politicising” the tragedy, looked to have doubled back on the remarks on Monday.
He said Khan - who was imprisoned for terror offences in 2012 and released halfway through his sentence last year - may have been “too tough to crack” in terms of rehabilitation.
“I’m just saying that in his case,” Mr Johnson told reporters, “I think that probably is true that people can’t be rehabilitated and I think it varies very widely.
“There are unquestionably some cases which are just too tough to crack and alas, he appears to have been one of them, and I’m afraid it was probably clear from the outset that that would be so.”
Asked what should be done with such offenders, Mr Johnson said: “I think you have to do what you can and a great deal of effort went in to try to change him and try to change his ways, but in the end better, I’m afraid, for the protection of the public, better for the protection of society and of us all, to keep him in [prison] rather than run the risk of letting him out.
“That’s what we’re proposing.”
Former Tory justice minister Phillip Lee, who has since defected to the Lib Dems, accused Mr Johnson of “lying and misleading” in the wake of the tragedy.
He told PA: “The desperate, sort of, politicisation of this by Boris Johnson – not a man who is known for details – wading into something which is actually quite complex is not appropriate.”
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The Tories sought to blame legislation passed under the last Labour government for the early release of Khan.
Labour in turn accused the Conservatives of starving the prison and probation services of resources, warning that it was impossible to “keep people safe on the cheap”.