William Barr, Donald Trump’s second attorney general, has been served with a subpoena in a $1.6bn defamation suit filed by a voting machine company against Fox News.

The subpoena appeared on the court docket for the lawsuit, which was filed by Dominion Voting Systems in Delaware in March 2021.

The suit concerns a false conspiracy theory, dismissed by Barr but advanced by the ex-president’s allies, that machines made by the company were tampered with in order to hand the Oval Office to Joe Biden.

The suit accuses the on-air personalities Maria Bartiromo, Tucker Carlson, Lou Dobbs, Sean Hannity, Jeanine Pirro “and their chosen guests” of spreading “defamatory falsehoods” about Dominion Voting Systems.

The suit alleges that “Fox knew these statements about Dominion were lies” but chose to “[sell] a false story of election fraud in order to serve its own commercial purposes, severely injuring Dominion in the process”.

It adds: “If this case does not rise to the level of defamation by a broadcaster, then nothing does.”

In a statement when the suit was filed, Fox News said the network “is proud of our 2020 election coverage, which stands in the highest tradition of American journalism, and will vigorously defend against this baseless lawsuit in court”.

Last month, a judge dismissed an attempt to have the suit dismissed.

Referring to the owners of Fox, the judge, Eric M Davis, cited “a reasonable inference that Rupert and Lachlan Murdoch either knew Dominion had not manipulated the election or at least recklessly disregarded the truth when they allegedly caused Fox News to propagate its claims about Dominion”.

News of the subpoena for Barr suggests the suit is moving into a higher gear. It follows a similar order for Brad Raffensperger, the Georgia secretary of state who resisted Trump’s attempt to overturn his defeat in that state.

Raffensperger and Barr have testified to the House committee investigating Trump’s attempt to overturn the election and the January 6 attack on the US Capitol.

In public hearings, the committee has played testimony in which Barr discusses the Dominion conspiracy theory and its effects on Trump.

Barr told investigators he found the Dominion claims “disturbing in the sense that I saw actually zero basis for the allegations but they were made in such a sensational way that they obviously were influencing a lot of people”.

Barr also said Trump’s claims about voter fraud were “bullshit”.

In office, Barr was widely seen as an attorney general willing to play defense for his president, to the extent of ignoring customary barriers between the White House and the justice department.

In summer 2020, the CNN media reporter Brian Stelter reported that Barr had told Rupert Murdoch to “muzzle” Andrew Napolitano, a Fox News legal analyst who had become critical of Trump.

A justice department spokesperson said that story was “false”.

Barr has said that if Trump is the Republican candidate for president in 2024, he will vote for him regardless of his assault on US democracy.