Prosecutors are in the process of issuing more than 100 subpoenas, sources say. The Justice Department in recent days has continued moving at an aggressive clip in ramping up its criminal investigation into the Alaska U.S. Senate race, sources familiar with the matter tell ABC News, with prosecutors in the process of issuing more than 100 subpoenas and at least twenty letters alerting individuals they are targets of the probe.

The investigation began accelerating last week after The New York Times detailed alleged efforts by Democrats and some in Democratic candidate Mary Peltola's campaign to recruit Daniel J. Sullivan, a retired teacher, to be on the ballot with incumbent Republican Dan S. Sullivan.

The target letters, which are being issued to Democrats whom investigators believe were involved in the recruitment effort, are an unusual step in such an early juncture in the investigation.

Among the recipients are Marc Elias, sources said, an attorney frequently attacked by Republicans over his extensive involvement in election-related litigation on behalf of Democrats.

"No one should be surprised that President Trump's Department of Justice is targeting me and my law firm on Election Day," Elias said in a statement. "This administration has repeatedly weaponized the DOJ against the President's political enemies despite zero evidence of wrongdoing, and we are the latest targets. We will not be intimidated by a sham investigation." A target letter, which is typically used by the Justice Department to alert an individual they are a "target" of a federal grand jury investigation, does not mean an individual will ultimately face federal charges.

According to sources, the letters indicate that a grand jury empaneled in Alaska is probing possible conspiracy and donor fraud related offenses. A DOJ spokesperson did not immediately respond to a request for comment from ABC News.