It started with threatening letters left at cell towers, then escalated to pipe bombs placed outside two cellphone stores.

Federal authorities allege a Michigan man took increasingly extreme actions — and warned that they could get worse — motivated by his unhappiness about “immoral content” on phones, including pornography and cursing.

John Douglas Allen, 75, was arrested Wednesday and charged with extortion and attempted damage or destruction of buildings used in interstate commerce. The Whittlemore, Mich., resident, a retired miner, faces up to 20 years in prison if convicted.

His attorney, Stevens J. Jacobs, could not immediately be reached for comment.

According to federal court documents, threatening letters began appearing at telecommunications towers across Northern Michigan on Aug. 25. Sealed inside polka-dot-decorated envelopes, they were sent from the “Coalition for Moral Telecommunication” and addressed to “AT&T, Verizon and all other Carriers.”

“We are prepared to travel throughout this Country and begin distroying [sic] inner city tower communication unless the following is followed,” one of the letters read. “All telecommunication containing immoral content must be stopped. This includes cursing, the transmission of pornography, and all manner of indecent communication.”

It also demanded $5 million and warned: “You may, if you like fight this, go to the Police, FBI, or any Government Agency. Understand this: If ONE of our people is arrested, now or in the future, your problems will begin.”

Investigators found three such letters at Northern Michigan cell towers — one in St. Ignace, another in Gould City and a third in Ontonagon Village.

Then, on the morning of Sept. 16, police went to a Verizon store in Cheboygan and an AT&T store in Sault Ste. Marie. Employees at both locations had discovered USPS boxes with wire coming out of them. The boxes were marked with the letters “CMT.” Each contained handcuffs. “Handcuff Johnny” was written on one and “HJ” on the other.

“In addition, both boxes contained language indicating that this was the ‘last warning,’ and that next time, it would be during ‘business hours,’ ” an arrest affidavit said.

The FBI’s explosives unit examined the boxes and determined that they were pipe bombs, one containing nails and the other metal BBs. If properly assembled and initiated, the agency said, the potential explosion could cause property damage, personal injury and/or death.

FBI officials obtained surveillance video footage that showed a man placing the boxes. They undertook what the agency described as “an exhaustive investigation” to identify the suspect.

They zeroed in on a vehicle the man was seen driving in the video, a maroon-colored Chevrolet minivan, tracking down its owner in part because of a dealership sticker on its back bumper. They visited the dealership and got information on similar vehicles it had sold.

One of them led them to Allen, who matched the characteristics shown on the surveillance footage, according to court documents. On Monday, FBI agents showed up at his home.

Allen admitted creating the letters and leaving them at cell towers, the affidavit said. He also allegedly said he had made the pipe bombs and left them at the stores.

“He described what he wrote on the boxes, including the reference to ‘Handcuff Johnny’ and the threat that next time, it would happen during business hours,” said the affidavit. “ ‘Handcuff Johnny’ referred to John Allen’s belief that he (i.e. ‘Johnny’) would end up in handcuffs as a result of his actions.”

He said he acted alone and there was no CMT group, describing taking action “because he became dissatisfied with all of the immoral content.”

Allen is in federal custody. He’s set to make his next court appearance Friday.

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