Federal prosecutors have launched an investigation into the attempt by Republicans in seven presidential battleground states won by Joe Biden in 2020 to subvert the election result by sending bogus slates of Donald Trump electors to Congress.

The ploy was one of the central tactics used by Trump loyalists as part of the “big lie” that he had defeated his Democratic challenger. The fake slates of electors were forwarded to congressional leaders, who then came under pressure to delay certification of Biden’s victory on 6 January 2021, the day of the Capitol insurrection.

In an interview on CNN, the deputy attorney general, Lisa Monaco, revealed that the justice department has begun an investigation into what she called the “fraudulent elector certifications”. She said the department had received referrals on the matter and “our prosecutors are looking at those”.

Monaco added: “We are going to follow the facts and the law wherever they lead to address conduct of any kind and at any level that is part of an assault on our democracy.”

Fake slates of Trump electors were sent to Congress from seven states in fact won by Biden – Arizona, Georgia, Michigan, New Mexico, Nevada, Pennsylvania and Wisconsin. Of those, two – New Mexico and Pennsylvania – added the caveat that the Trump electors should only be counted in the event of a disputed election.

The other five states sent signed statements to Washington giving the appearance that Trump had won despite clear and verified counts placing Biden on top.

Under America’s arcane presidential election system, US presidents are not chosen directly by voters but indirectly through electoral college votes meted out state by state. Official certificates naming the electors for the winning candidate in each state are then sent to Washington to be certified, in this case on 6 January, when hundreds of violent Trump supporters stormed the Capitol building in an attempt to disrupt the process.

Earlier this month the pro-democracy group American Oversight obtained under freedom of information laws the bogus certificates from all seven states in which Republicans attempted to overturn the election result. The certificate from Georgia, one of the most hotly contested states in 2020, reads: “We, the undersigned, being the duly elected and qualified electors for president and vice president of the United States of America from the state of Georgia …”

The fake statement then carries the names and signatures of 16 fake electors who claimed falsely to have cast their electoral college votes for Trump when in fact they had no legal standing to do so. The move was in direct contravention to the actual vote in Georgia, confirmed in multiple counts, which Biden won by 11,779 votes.

Democratic attorneys general in at least two of the seven states – New Mexico and Michigan – have now asked federal prosecutors to examine whether drawing up the bogus certificates amounted to a crime. Their referrals appear to have triggered the DoJ’s investigation.

The fact that Republicans left a paper trail by sending their phony certificates to both Congress and the National Archives suggest that they may now face legal peril. The House committee investigating the January 6 insurrection has also recently begun to focus on the fake Trump electors, and particularly those who organized the plot.

A figure of special interest is Rudy Giuliani, who acted as a lawyer for the Trump campaign and who has been reported to have spearheaded the fake elector strategy. The January 6 committee sent Giuliani a subpoena letter earlier this month specifically referring to his efforts instigating the ploy.

Another area of intense interest is the draft letter prepared in December 2020 by Jeffrey Clark, a relatively lowly justice department official, who tried to persuade Georgia and six other states won by Biden to call back their electors from Congress and consider replacing them with Trump electors. The letter was never officially sent after the acting US attorney general, Jeffrey Rosen, refused to play ball.

The fake electors tactic was also central to the election subversion strategy laid out for Trump by the conservative lawyer John Eastman. In a now notorious two-page memo handed to Trump and the then vice-president, Mike Pence, in the Oval Office, Eastman argued that Pence could block the certification of Biden’s victory on 6 January.

Pence had the constitutional role of presiding over the joint session of Congress that would certify the election results – a process usually considered purely ceremonial. But Eastman advised him that when he opened the electoral college ballot from Arizona he should announce that “he has multiple slates of electors, and so is going to defer decision on that”.

By “multiple slates”, Eastman was referring to the official slate of electors returned by Arizona in favor of Biden who won the state by 10,457 votes and the fake slate of Trump electors that is now under federal investigation.