Mulligan was terminated July 15.
The federal investigation is ongoing in Pocahontas, with the help of the Arkansas State Police.
Mulligan was suspended after an FBI investigation in May that resulted in one arrest of a subject who's name has still not been released.
The Mayor and city council said the results of the investigation may have a negative effect on the city and therefore took action, firing Mulligan.
Mulligan has the right to appeal the decision and has not been charged with a crime.
Tackett has been with the department since 2005.
House fire claims two
Pocahontas police say the deaths of a man and woman whose bodies were found in a burned home are not being investigated as possible homicides.
The bodies of Nancy Hafner and her boyfriend, George Jackson, both in their mid-50s, were found shortly after midnight on Sunday, July 31, in their home at 709 Mason.
After the fire was extinguished, the bodies were sent to the State Crime Laboratory in Little Rock for positive identification and to determine the cause of death.
Interim Police Chief Cecil Tackett said the cause of the fire has not been determined, although it does not appear to be arson.
"It was an old house, probably 65 to 70 years old," Tackett said Monday, Aug. 8.
The fire likely started in wiring. The couple died of smoke inhalation, he said.
The incident is not being investigated as a homicide, he said, contrary to other news reports.