Story highlights Sebastian Gregerson was charged with possessing a destructive device and receiving explosive materials with a license

The affidavit reveals the FBI began physical surveillance of Gregerson as early as January

Washington (CNN) A 29-year-old Detroit man was arrested by the FBI after allegedly attempting to buy grenades in addition to amassing a large arsenal of weapons, ammunition, tactical gear and training material "unlikely" to be used recreationally, according to a criminal complaint filed Monday.

Sebastian Gregerson, also identified by the FBI as Abdurrahman Bin Mikaayl, was arrested on Sunday and charged with unregistered possession of a destructive device and the unlicensed receipt of explosive materials. The charges follow a 16-month investigation by the FBI, which was initiated after a confidential source "alerted the FBI that Gregerson claimed to have in his possession both grenades and bazookas," according to court documents.

The criminal complaint did not allege a motive for attempting to procure illegal explosives, but Gregerson described at one point to the undercover FBI employee "his plans for a full tactical response to law enforcement if they came for him, including the use of grenades," according to court documents.

Gregerson is due back in court for a detention hearing on Thursday. A message left with Gregerson's attorney was not immediately returned Tuesday.

According to the affidavit, Gregerson, over a period of eight months, had purchased 700 rounds of ammunition for an AK-47, tactical gear and clothing, videos used by military and law enforcement for tactical firearms techniques, military-grade splints for medical care, commercial-grade road spikes and other hardware typically used for combat training.

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