MANITOWOC, Wis. (AP) — Bomb threats to a Wisconsin county sheriff's office that made an apparent reference to "getting justice" for the convict profiled in the popular Netflix series "Making a Murderer" are the latest in a long string of threats made to the department and its officers since the series went online, officials said Thursday.

Four of the dozens of phone and email threats made to the Manitowoc County Sheriff's Department have been referred to the FBI and the Wisconsin Department of Justice because they include specific targets, including individual officers, sheriff's Inspector Gregg Schetter told The Associated Press.

Law enforcement has asked the more than 100 employees at the sheriff's department and jail to be vigilant for possible trouble, Schetter said.

"It's hard for them to know whether their safety is guaranteed," he said. "It's been unnerving to say the least."

The latest threats were made by a male caller Wednesday evening, warning of bombs inside the sheriff's office building and a vehicle in the parking lot "packed with explosives." A second, "very similar" threat was received about 20 minutes later, according to Manitowoc police. A search did not turn up anything suspicious.

The caller also mentioned "getting justice for Steven," something Manitowoc police said was an apparent reference to Steven Avery, the Wisconsin man profiled in the 10-part Netflix series released in December who was convicted of killing photographer Teresa Halbach a decade ago.

The series implies the Manitowoc County sheriff's deputies planted evidence in the case, a claim authorities have dismissed.

Avery had been wrongfully convicted years earlier in a rape case and served 18 years in prison. He sued Manitowoc County for tens of millions before he and his nephew, Brendan Dassey, were convicted in Halbach's death.

"It's troubling that people talk about wanting fairness in this case, yet are so quick to make decisions and bring violence into it as a solution," Schetter said.

Extra patrols have been added around the sheriff's department, the dispatch center, the jail and courthouse, he said.

"I know it only takes one person to cause harm. I'm afraid for our employees and the citizens in our city and our county," he said. "I pray that nobody acts out on anything based on a film."