By Laila Kearney

(Reuters) - Federal agents have arrested an Oklahoma man accused of mailing a package bomb last year addressed to Joe Arpaio, the controversial Arizona lawman who calls himself "America's toughest sheriff," officials said on Tuesday.

Gregory Lynn Shrader, 55, is suspected of dropping off the parcel, filled with gunpowder and other materials, at a U.S. Postal Service mailbox in Flagstaff, Arizona, but the package was intercepted by state authorities before it reached Arpaio, according to the U.S. Postal Inspection Service.

The device was later rendered harmless by police.

Shrader was arrested last week and is being held without bond at the Tulsa County Jail, where he is awaiting release to the custody of U.S. marshals, according to the county's website.

He was charged in a criminal complaint with one count of willfully making a threat through the U.S. mail.

Arpaio, the sheriff of Maricopa County, has drawn criticism for his aggressive stance on illegal immigration. He was the subject of a U.S. Justice Department probe over his office's treatment of Hispanics and has been scrutinized for housing county inmates in a Spartan "Tent City" jail.

He has also been the subject of multiple threats of violence.

"I understand that with being a high-profile sheriff, sometimes they don't like me, but they don't have to blow me up and make all these vicious overtures," Arpaio told Reuters on Tuesday.

In an incident unrelated to Shrader's case, a 15-year-old boy in Canada was arrested on Tuesday on suspicion of threatening to kill Arpaio five times over the past eight months, the Maricopa County Sheriff's Department said.

Arpaio, who is considering running for Arizona governor in November, said the threats have encouraged him to "fight harder" to do his job. "Nothing's going to scare me away," he said.

The Phoenix branch of the U.S. Attorney's Office, which is prosecuting Shrader, was not immediately available for comment.

The Federal Bureau of Investigation and the Postal Inspection Service led the investigation into the package bomb, according to statement released by the agencies.

Federal agents arrested Shrader after conducting a search of his home in the small city of Jay, Oklahoma, on Thursday and finding a document with an image that closely resembled the return address label on the package in question.

Interviews with potential witnesses and video footage of the area where the package was mailed led authorities to Shrader, officials said.

(Reporting by Laila Kearney in San Francisco; Editing by Steve Gorman and Lisa Shumaker)