The Justice Department is seeking to preserve and separate Alaska ballots that rank independent Senate candidate Daniel J. Sullivan, as federal prosecutors investigate whether his name on the ballot caused voters to confuse him with Republican Sen. Dan Sullivan.

The request came Friday, just days before Alaska's ranked-choice general election. The contest features the two-term Republican senator and Democrat Mary Peltola, a former congresswoman, in a race that could help determine control of the U.S. Senate.

In a separate development, U.S. Attorney Michael Heyman confirmed in Anchorage that federal authorities have been conducting a criminal investigation into the circumstances surrounding Daniel J. Sullivan's candidacy for several months. Heyman said recent reporting about efforts to recruit the retired teacher had prompted a "significant surge of law enforcement assets to Alaska."

Prosecutors have not identified the potential crimes under investigation or disclosed who may be targeted. Heyman declined to provide those details but encouraged members of the public to submit information that could assist investigators.

The Justice Department's civil lawsuit seeks to prevent certification of the Senate election if the Republican incumbent loses. In its latest court filing, Assistant U.S. Attorney E. Bryan Wilson asked a judge to require Alaska election officials to preserve and segregate every ballot ranking Daniel J. Sullivan. The government says the ballots may need to be examined to determine whether voters mistakenly selected the independent candidate believing they were voting for the incumbent.

Federal attorneys argued that the similar names create a "substantial risk" of misleading voters and undermining confidence in the election process. They requested a ruling by Monday and also submitted proposed questions they want the court to require Peltola's campaign and the Democratic Senatorial Campaign Committee to answer.

Alaska officials have disputed the need for such an intervention. The state says it has already taken precautions to distinguish the candidates on the ballot. Republican Sen. Dan Sullivan is identified as Dan S. Sullivan, with his party affiliation and incumbent status listed. The independent candidate appears as Daniel J. Sullivan Jr. without a party designation.

State attorneys have also pointed to Alaska's ranked-choice voting system, which permits voters to rank both candidates rather than forcing them to choose only one. The two men appeared under the same identifying information during the state's August open primary.

The controversy intensified after The New York Times reported that Peltola's campaign appeared to have played a role in recruiting the retired teacher to enter the race. Peltola denied knowing about the effort, telling supporters at a Thursday campaign event in Juneau that she knew of no other candidates beyond herself and the incumbent.

"I am not interested in tricking voters," Peltola said.

Democrats, including the DSCC, have denied involvement in any effort to manipulate the ballot, while Daniel J. Sullivan has also rejected claims that he was recruited to run on behalf of Democratic interests.

President Donald Trump has said he directed the Justice Department to pursue the civil case. Meanwhile, Republican Gov. Mike Dunleavy announced an investigation into whether any violations occurred after the reporting about the candidate's recruitment became public.

The Justice Department's request in Alaska follows a separate setback in California, where the Ninth Circuit Court of Appeals rejected the department's effort to have disputed ballots set aside. That case involved roughly 20,000 people the administration contends may not be U.S. citizens. California officials challenged the federal list, arguing it was unreliable and likely included many eligible American citizens.

The public confirmation of the Alaska criminal investigation has also drawn attention because federal prosecutors traditionally avoid announcing investigative developments close to an election. The timing is particularly sensitive with voting approaching and the Justice Department pursuing a separate lawsuit that could affect certification of the Senate result.