The FBI said two explosive devices recently found outside cellphone stores in northern Michigan are believed to be linked to letters found last month in the Upper Peninsula.

The bombs, which were discovered and defused Thursday, were found in suspicious packages left outside a Verizon store in Cheboygan and an AT&T store in Sault Ste. Marie. The FBI and Michigan State Police said in a statement that they believe the devices are related to a series of letters discovered in the Upper Peninsula last month.

The letters claimed to be from the “Coalition for Moral Telecommunications” and made demands of telecommunications companies. They were found at several telecommunications tower sites, according to the FBI.

Cheboygan authorities said emergency personal were dispatched around 9 a.m. Thursday after an employee reported a suspicious package near the door of the business. A bomb was found inside, which authorities removed and defused. A similar device was reported in Sault St. Marie.

The FBI said both devices were found in U.S. Postal Service boxes which were sealed with black duct tape and placed outside the stores. Each one had “CMT” written on it and contained threatening notes addressed to the cellphone companies.

Multiple law enforcement agencies were investigating.

(AP)