John Ferak

USA TODAY NETWORK-Wisconsin

The FBI opened an investigation into threatening email messages and a phone call from a man pretending to be an FBI agent several weeks after the debut of the "Making a Murderer," but no criminal charges have resulted.

Manitowoc County Sheriff Rob Hermann sensed that one email sent to him on Jan. 16 crossed the line between been being angry and threatening harm.

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The email stated: "(expletive) pigs I hope you rot in hell. I live far from WI, but if I ever visit your (expletive) area of America, I will knock out one of your coward cops and James Lenk. Guy better watch his back. Avery might be guilty but I have never seen police corruption so evident like in that case. Watch your backs ... I will taste your (expletive) blood and your families too."

The Manitowoc sheriff reached out to longtime FBI Special Agent Gerald Mullen about the matter.

"I know Lt. (Andrew) Colborn has been forwarding some information to you regarding the Avery case," Hermann wrote. "Here is a recent threatening email that I received."

Mullen, who is based at the FBI's offices in Green Bay, determined that the Jan. 16 email might be a violation of federal law.

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"The decision to prosecute will be at the discretion of the US Attorneys Office," Mullen advised. "I plan on opening a threat case and will get a subpoena to identify the subject who sent you this email. He will be receiving a visit from the FBI for an admonishment.

"I believe we can make a strong case for a prosecution due to all of the publicity the Netflix video has generated. If people realize they may be charged with a federal felony, they may think twice before threatening law enforcement. I will keep you posted."

Hundreds of emails sent to Hermann and other Manitowoc County officials in December and January were recently turned over to the USA TODAY NETWORK-Wisconsin to comply with a public records request. The news organization sought access to written or electronic correspondence involving several current law enforcement officials at the Manitowoc County Sheriff’s Office.

Timeline: History of the Steven Avery case

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Since December, people around the world have expressed outrage and strongly criticized Manitowoc County law enforcement's role in convicting Steven Avery of killing photographer Teresa Halbach, but the vast majority of these angry emails were not threatening.

The emails turned over to the USA TODAY NETWORK-Wisconsin also revealed an FBI impostor tried to dupe the Manitowoc County Sheriff's Office.

Manitowoc County emergency dispatcher Kathy Greenwood took the phone call around 1 p.m. on Jan. 19 from a man wanting to talk to internal affairs.

Greenwood asked him a series of questions to determine where to transfer his call because she realized the sheriff's office does not have an internal affairs division, she stated in her email.

Meanwhile, the unidentified man grew verbally abusive and irate, Greenwood noted in an email to her supervisors. "I did disconnect the call as we had busy radio traffic and numerous other phone calls coming in," she stated.

Minutes later, the man called back again.This time, he claimed to be "Agent Carter with the FBI based in Washington, D.C."

"I again, I realize now mistakenly, engaged in conversation with him attempting to find out who I needed to direct him to as he was now asking for a captain or a sergeant but refusing to give me any idea what his call was in reference to," Greenwood wrote. "I was assuming it was in reference of the Avery case."

Ultimately, the Manitowoc dispatcher transferred the call into the voice mail system of Undersheriff Gregg Schetter. That afternoon, Schetter tried calling the phone number back. The man answered saying "hello."

"I asked to speak with Agent Carter and he (started) speaking," Schetter stated. "I confirmed again asking 'Is this Agent Carter from the FBI? And he said 'yes it is.' I asked for his first name and he asked why I was calling?"

Schetter explained that someone from the caller's phone had recently called into county dispatch identifying himself as an FBI agent wanting to speak with internal affairs.

After Schetter asked how he could assist, "the male sounds nervous and tells me he has contacted the Governor and a Senator already and it has been handled," according to the email records.

Finally, Schetter told the man he still needed to confirm his identity with the FBI before he released the phone call. The caller suddenly hung up on him, Schetter stated.

"I strongly doubt he is an agent, and they may like to know who is impersonating them to other (law enforcement) agencies," Schetter advised the sheriff.

Eventually, Hermann forwarded the email along to FBI agent Mullen. "This call may interest you also," Hermann stated.

John Ferak: 920-993-7115 or jferak@gannett.com; on Twitter @johnferak