In the letter, Austin promised a federal lawsuit and took issue with Arpaio's "episodic insistence" for more evidence of civil rights abuses. In December, the DOJ accused Arpaio's office of carrying out a pattern of discrimination against Latinos, including racial profiling.
The two sides had planned to meet Wednesday to discuss ways to resolve the racial profiling allegations. The meetings were called off.
Appointment of an outside monitor would transfer much of the decision-making power of Sheriff Arpaio's office to a person or group of people selected by the federal government. The monitor would need to approve policies, operational decisions, and new enforcement initiatives.
The move nullifies the will of Maricopa County voters who elected Sheriff Arpaio, according to a statement from the sheriff.
"I am the constitutionally and legitimately elected sheriff, and I absolutely refuse to surrender my responsibility to the federal government," Sheriff Arpaio said. "And so to the Obama administration, who is attempting to strong arm me into submission only for its political gain, I say, 'This will not happen, not on my watch.'"