Washington (CNN) -- Federal officials fired a new shot Monday in the ongoing battle between the Obama administration and local law enforcement in Maricopa County, Arizona.
The Justice Department expressed frustration at the actions of Maricopa County Attorney William Montgomery, saying he has been a moving target and provided inconsistent comments on his relationship with the office of Sheriff Joseph Arpaio, who is under federal investigation.
In a letter to Montgomery, Deputy Assistant Attorney General Roy Austin, Jr. said he was particularly upset over the prosecutor's stance on sex crimes in the key Arizona county, which includes Phoenix.
In December, the Civil Rights Division of the Justice Department said its investigation determined "there are an alarmingly large number of cases involving serious allegations of sex crimes where the MCSO (Maricopa County Sheriff's Office) either did no investigation or a wholly inadequate investigation."
Montgomery responded during a news conference on April 4, saying he had "not seen anything in the nearly 18 months as Maricopa county attorney to support the systemic allegations the Department of Justice is making."
The Justice Department told Montgomery an independent monitor needs to be appointed by a federal court to ensure the necessary reforms are "real, comprehensive, and sustainable."
The letter does not mention a possible lawsuit on the matter, which the Civil Rights Division could file against the sheriff's office on the issue.