The former special counsel Robert Mueller made a rare move on Saturday to publicly defend his two-year investigation into allegations of collusion between the Trump campaign and Russia in the 2016 election – and to castigate Donald Trump’s decision to commute Roger Stone’s prison sentence.

Mueller wrote an opinion article for the Washington Post [paywall] published under the headline “Roger Stone remains a convicted felon, and rightly so”.

“The work of the special counsel’s office – its report, indictments, guilty pleas and convictions – should speak for itself,” he wrote.

“But I feel compelled to respond both to broad claims that our investigation was illegitimate and our motives were improper, and to specific claims that Roger Stone was a victim of our office …

“Stone was prosecuted and convicted because he committed federal crimes. He remains a convicted felon, and rightly so.”

Trump commuted the sentence of Stone on Friday night, sparking outrage from Democrats and some senior Republicans.

Stone was a former campaign adviser to the president, convicted in November 2019 of seven crimes including obstruction of justice, lying to Congress and witness tampering.

The 2017-19 Mueller investigation uncovered evidence of communications between Stone and WikiLeaks related to the release of hacked Democratic party emails during the 2016 election, discovered in a separate inquiry into Russian intelligence officers charged with hacking the emails and staging their release.

The partially released Mueller report in April 2019 described Russian efforts to tamper with the election and the Trump campaign’s receptivity to certain “Russian offers of assistance to the campaign”.

It outlined actions by Trump that may have amounted to obstruction of justice and concluded: “While this report does not conclude that the president committed a crime, it also does not exonerate him.”

Mueller also concluded he did not have the power to charge Trump even if he thought it was warranted.

Mueller wrote: “The special counsel’s office identified two principal operations directed at our election: hacking and dumping Clinton campaign emails, and an online social media campaign to disparage the Democratic candidate.

“We also identified numerous links between the Russian government and Trump campaign personnel – Stone among them. We did not establish that members of the Trump campaign conspired with the Russian government …

“The investigation did, however, establish that the Russian government perceived it would benefit from a Trump presidency and worked to secure that outcome. [And] that the campaign expected it would benefit electorally from information stolen and released through Russian efforts.”

Trump has repeatedly attempted to discredit Mueller and his investigations.

Mueller has kept his counsel since he testified in Congress in July last year. It was a muted affair, and many perceived Trump was emboldened in his efforts to seek assistance in his current election campaign from the Ukraine.

This led to the historic impeachment of the president, and Trump’s ultimate acquittal by the Senate earlier this year.

On Saturday Mueller wrote: “Russia’s actions were a threat to America’s democracy. It was critical that they be investigated and understood.”

‘Historic corruption’ – Romney

Republicans largely stayed silent on the issue on Saturday, however Utah Senator Mitt Romney, who was also the lone GOP senator to vote to convict the president during his impeachment trial earlier this year, attacked Trump’s move.

“Unprecedented, historic corruption: an American president commutes the sentence of a person convicted by a jury of lying to shield that very president,” Romney tweeted.

Mark Sanford, the former South Carolina congressman who made a short-lived primary challenge to Trump, tweeted: “So much for the Republican Party being the party of law and order. Have we not lost our minds in not condemning as a party the presidents pardon of corruption by Roger Stone”.

But most of Republicans who did speak out about the decision supported it. Senator Lindsey Graham, a Trump confidant, said Stone was convicted of a nonviolent, first-time offense and the president was justified in commuting the sentence.

• Associated Press contributed to this report