Federal prosecutors served six grand jury subpoenas Tuesday to Minnesota officials as part of an investigation into whether they obstructed or impeded federal law enforcement during a sweeping immigration operation in the Minneapolis-St. Paul area, a person familiar with the matter said.

The subpoenas, which seek records, were sent to the offices of Gov. Tim Walz, Attorney General Keith Ellison, Minneapolis Mayor Jacob Frey, St. Paul Mayor Kaohly Her and officials in Ramsey and Hennepin counties, the person said.

Walz and Frey, both Democrats, have called the probe a bullying tactic meant to quell political opposition. Frey’s office released a subpoena, which requires a long list of records for a grand jury on Feb. 3, including “cooperation or lack of cooperation” with federal authorities and “any records tending to show a refusal to come to the aid of immigration officials.”