The suit raises the prospect of a messy fight over a contest that could determine Senate control in next Tuesday's midterm elections.
Alaska is one of about 10 highly competitive races that will decide control of the Senate, where Republicans hold a 53-47 majority. Democrats need a net gain of four seats, and a post-election dispute in Alaska could leave control of the chamber unclear for weeks.
Alaska Governor Mike Dunleavy also called for a state investigation into the allegations, after the New York Times reported that aides to Democratic candidate Mary Peltola worked to recruit two candidates who could siphon votes from the incumbent Senator Sullivan.
"They orchestrated his sham campaign for the purpose of misleading Alaskan voters and getting Peltola elected," lawyers for the Justice Department wrote in their amended complaint, citing the Times report.
Peltola's campaign and its lawyers did not immediately respond to a request for comment on the Justice Department's amended complaint.
Reuters has not verified the New York Times report.
Trump for years has sowed doubts about the fairness of US elections and even prior to the Times report had pushed the Justice Department to file a lawsuit seeking to pre-emptively block Alaska from certifying Peltola, a former congresswoman, if she were to win the election.
The Justice Department issued more than 40 subpoenas and document preservation requests in its probe, a source familiar with the matter told Reuters on Thursday.
President Donald Trump told reporters earlier on Thursday: "It's hard to believe that she could even be allowed to run. Frankly, I don't know how you can allow somebody like that to run. It's so unfair."
As campaigning enters its final few days, the White House has shelved plans for Trump to appear on Saturday in Kansas, the site of an unexpectedly competitive Senate campaign, a Republican campaign official said.
"I don't feel like I have to go to Kansas," Trump told reporters.
Instead, the president will participate in more than 20 virtual campaign appearances before Tuesday, a White House official said.
"These events give him the opportunity to highlight his accomplishments and expand his reach across the country," the official said.
The Times reported Peltola aides worked with campaign lawyers and staff at the Democratic Senatorial Campaign Committee to recruit two candidates who could siphon votes from the incumbent Republican Sullivan, including a retired teacher named Daniel J. Sullivan Jr., who is on the ballot without a party affiliation.
Peltola's former campaign manager, Elisa Rios, who was fired in August, told the Times that Peltola was aware of the effort.
Peltola's campaign said it "has no involvement with either Sullivan campaign."
The DSCC also denied involvement and accused Republicans of propping up candidates of their own in other races.
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