A source, who spoke on the condition of anonymity on Friday, said subpoenas were prepared for Walz and Frey as part of the inquiry, but it was not immediately clear whether they had been served.
The investigation, first reported by CBS News, stems from statements made by Walz and Frey about the thousands of Immigration and Customs Enforcement officers and Border Patrol agents deployed to the Minneapolis region in recent weeks under orders from President Donald Trump, the source said.
Reacting on social media to news of the investigation, Walz, who unsuccessfully ran for the vice presidency in the 2024 election won by Trump, said the federal justice system was being weaponised to intimidate Trump's perceived political enemies.
"Two days ago it was Elissa Slotkin. Last week it was Jerome Powell. Before that, Mark Kelly. Weaponising the justice system against your opponents is an authoritarian tactic," Walz said.
The governor was referring to US Senators Elissa Slotkin and Mark Kelly, Democrats from Michigan and Arizona, who made a video statement urging members of the military to resist illegal orders, and Federal Reserve Chairman Jerome Powell, whom Trump has criticised as being too hesitant to raise interest rates.
Reacting to a CNN report on the investigation, Frey said separately: "This is an obvious attempt to intimidate me for standing up for Minneapolis, local law enforcement, and residents against the chaos and danger this Administration has brought to our city."
The Justice Department declined to comment. But US Attorney-General Pam Bondi posted a message on X on Friday evening, saying: "A reminder to all those in Minnesota; No one is above the law."
It would be highly unusual for federal prosecutors to bring a criminal conspiracy case based on statements from public officials about government policies.
The Trump administration has sent nearly 3000 federal agents to Minnesota since early last week, triggering angry protests in Minneapolis over the surge in immigration agents on the streets of the state's most populous city.
Confrontations between residents and federal officers have become increasingly tense after an ICE agent fatally shot a US citizen, Renee Good, 37, behind the wheel of her car, in Minneapolis on January 7, triggering daily protests that have spread to other cities.
Although he has urged protesters to remain peaceful, Walz also has encouraged citizens to record video of any arrests or other encounters between ICE agents and members of the public in order to create a database for potential "future prosecution" of wrongdoing by law enforcement.
Walz and Frey have repeatedly demanded that Trump end the ICE deployment, saying the militarised deportation drive was a reckless operation aimed at sowing chaos, fear and headlines, and was putting civilians in danger.
Deputy US Attorney-General Todd Blanche earlier this week blamed Walz and Frey for the confrontations and said he was focused on stopping them by "whatever means necessary.
