PHOENIX (AP) - Authorities are investigating an explosive device addressed to Arizona's Sheriff Joe Arpaio, the self-proclaimed "toughest sheriff in America" known for his strict treatment of jail inmates and cracking down on illegal immigrants.

The device intercepted late on Thursday was in a package addressed to Sheriff Arpaio at his downtown Phoenix office, Sheriff Arpaio's office said in a statement.

It appeared suspicious, so it was X-rayed and the device was detected. A bomb squad team neutralised the explosive, the statement said.

Sheriff Arpaio's chief deputy, Jerry Sheridan, told The Arizona Republic that investigators believe the package was picked up on Thursday at a rural Post Office box.

Following the killing of a West Virginia sheriff last week, Sheriff Arpaio said elected law enforcement officials across the nation seem to be targeted.

A national hero to conservatives on immigration issues, Sheriff Arpaio himself has been the target of numerous threats. That prompted the need for a security detail for the lawman also known for dressing jail inmates in pink underwear and making them sleep in tents in the heat of the Arizona desert.

A campaign to recall Sheriff Arpaio began just weeks after he started his sixth term in January.

Critics contend Sheriff Arpaio should be ousted because his office failed to adequately investigate more than 400 sex-crimes cases, allegedly racially profiled Latinos in its trademark immigration patrols and has cost the county US$25 million (S$31 million) in legal settlements over treatment in county jails.

Sheriff Arpaio has denied that his deputies racially profiled Latinos in traffic patrols targeting illegal immigration. He has said his office has moved to clear up the sex-crime cases and moved to prevent the problem from happening again.