(CNN) -- An Arizona lawman said the suspicious package addressed to controversial Sheriff Joe Arpaio "would have caused a major explosion" if someone had opened it.
Jerry Sheridan, chief deputy of the Maricopa County Sheriff's Department, told reporters Friday the package was noticed by an alert postal employee in Flagstaff who thought he spotted gunpowder residue. Arpaio was at Sheridan's side as he spoke.
Police in Flagstaff X-rayed the package, noticed what appeared to be an explosive device, and rendered it safe by blowing it up with a water cannon.
Sheridan said the package was mailed Thursday, a day after the department received a death threat from a major drug cartel.
The department has received other threats in the past, but, Sheridan said, this development is "something that was brought to the height of seriousness."
"That is a very serious threat. It was labeled to come here, to this building and to be opened by Sheriff Arpaio," Sheridan said.
He said injuries, burns and maybe death could have resulted. The FBI and deputies were trying to track down "a person of interest."
Arpaio, well-known for his hardline anti-immigration policies that have led to accusations of civil rights violations, vowed that he would not be brought down by his enemies. He spoke after Sheridan briefed reporters.
He said he's been besieged by certain groups and people and has been slammed with rhetoric criticizing him. He said he's resolved to do his job and won't be intimidated by people who want to force him out of office or kill him.
"My answer is it will not deter me from doing my job," he said. "I'm not running scared.
"All I know is I'm going to keep being the sheriff," he added. "I want everybody to know that."
Earlier, sheriff's department spokeswoman Lisa Allen said it was her understanding that the package had gun powder, wires, a battery -- all the components for an explosion.
U.S. Postal Inspector Andrew Rivas in Flagstaff, who screened the package Thursday, considered it suspicious enough to call a local police bomb squad and the FBI.
"We evacuated the post office, got all our employees to safety," Rivas told CNN affiliate KTVK.
Rivas said authorities have an idea of where the package may have come from, but declined to specify, citing the ongoing investigation. Allen said the origin of the package is one of the underlying issues in the investigation.
At present, the department is not opening mail and is checking to see if any suspicious packages have made it through, Allen said. Neither she nor Flagstaff police knew if the box contained a message.
The FBI declined to release details.
"I can confirm the FBI is involved in the investigation," said FBI spokesman Manuel Johnson in Phoenix. "At this time the FBI is not commenting further due to the matter being ongoing."
The Bureau of Alcohol, Tobacco, Firearms and Explosives also is investigating, ATF spokesman Mike Campbell said.
Flagstaff is about 150 miles north of Phoenix, the seat of Maricopa County.
A lightning rod for controversy
Arpaio has made national headlines for years with his unorthodox -- and often controversial -- style of justice. And he has been the object of threats before.
Since August 2011, nine threats have been directed at Arpaio that were were credible enough to be investigated. There has been at least one arrest in the past involving threats.
While Arpaio already has reasonable security, Allen said, "we are going to have to augment" it either by getting more personnel or adding electronic security.
Arpaio has housed thousands of inmates in tents and forced all inmates to wear pink underwear. He has boasted about feeding each inmate on less than $1 a day.
His critics say he has a long history of launching bogus criminal investigations against political opponents and anyone else who gets in his way.
He was the subject of a civil lawsuit by the Justice Department alleging civil rights violations. According to the complaint, the sheriff's office has displayed a pattern of discrimination against Latinos that includes racial profiling, unlawful detention and searches, and unlawful targeting of Latinos during raids.
Arpaio has denied any discrimination, and one of his attorneys called the Justice Department investigation a "witch hunt."
His office website touts his "get tough" policies and says his chain gangs contribute thousands of dollars of free labor to the community. Male chain gangs, as well as the world's first-ever female and juvenile chain gangs, clean streets, paint over graffiti and bury the indigent in the county cemetery.
After winning his sixth term last November, the 80-year-old sheriff said he doesn't plan on leaving office anytime soon.
"For my critics out there, I'm going to say right now: In January, I'm signing up for 2016. So I'm not a lame duck," he told a crowd of cheering supporters.
CNN's Greg Morrison, Ashley Fantz and Michael Martinez contributed to this report.