The U.S. Justice Department told Maricopa County Sheriff Joe Arpaio that it would investigate his department over allegations of discrimination and unconstitutional searches and seizures.
The Justice Department letter offered no specific allegations.
But Arpaio said he thought the investigation was spurred by his department's often controversial efforts to combat illegal immigration in Maricopa County, which includes Phoenix.
Arpaio has launched crime sweeps in areas with high concentrations of Latinos.
He has also started to separate illegal immigrants from other inmates in Tent City, a section of the county jail where all inmates are housed in tents.