Massachusetts mailed letters to all licensed gun owners in the state on Thursday demanding they surrender any bump-fire stocks or other newly banned firearms accessories to police.

The Massachusetts Executive Office of Public Safety and Security, which oversees the state's registry of Firearms Identification cards, said it had begun mailing a letter to all registered gun owners in the state warning that once the state's ban on bump-fire stocks, trigger cranks, and other firearms accessories designed to help increase a semi-automatic firearm's rate of fire takes effect on Feb. 1, 2018, they will be forced to surrender those devices or face prosecution. The agency said the letters have already been sent and Massachusetts gun owners should be receiving them shortly.

"It went out snail mail so license holders should be receiving as we speak [or] in the coming days," Felix Browne, a spokesperson for the Executive Office of Public Safety and Security, told the Washington Free Beacon.

The agency said there is no registry of bump-fire stocks, and authorities don't know how many bump-fire stocks are in the state or who owns them. Browne said the letters were being sent to registered gun owners because the bump-fire stock ban required they be notified of the new regulations.

"Effective 90 days from the enactment of the bill—February 1, 2018—the new law will also prohibit possession of bump stocks or trigger cranks, including possession in a private home," the letter reads in part. "There are no exceptions to this prohibition for licensed firearm owners: an FID card, a License to Carry, or even a license to possess a machine gun will not authorize possession of a bump stock or a trigger crank."

The letter warns that those who currently own a bump-fire stock or other newly prohibited item must turn them into police by May 2018 or face arrest and prosecution.

"Because the law does not allow for transfer or sale of these prohibited items, if you currently possess a bump stock or trigger crank within the Commonwealth of Massachusetts you should contact your local police department or the Massachusetts State Police to get details about how to transfer custody of the prohibited item to the police for destruction," the letter said. "Retention of such a prohibited item beyond the 90 day grace period will expose the owner to criminal prosecution."

Bump-fire stocks have come under scrutiny since a shooter used rifles equipped with the devices to kill dozens in Las Vegas last October. In November, Massachusetts became the first state to explicitly ban the devices and New Jersey followed suit this week. Both states currently employ some of the nation's strictest gun-control regimes.

The Bureau of Alcohol, Tobacco, Firearms, and Explosives has initiated a review of its 2010 ruling that classified bump-fire stocks as firearms accessories which aren't subject to federal gun-control laws, a move supported by the National Rifle Association and other gun-rights groups. The NRA and other gun-rights groups have opposed federal legislation seeking to ban and confiscate bump-fire stocks and other firearms accessories.

The Massachusetts-based gun-rights group Gun Owners' Action League (GOAL) called the letter to Massachusetts gun owners "a doozy." They called it a "veiled threat" and complained about the lack of key details.

"There is no mention as to how an individual who chooses to comply with this law will be able to show proof that they did," the group said in a post on its website. "There is no mention about what the police will do with these devices upon receiving them."

The group said it is weighing whether or not to file suit against the state over the law.

"At this time GOAL is weighing options as to what is the best course of action going forward," the post said. "In our opinion this is an illegal and unconstitutional taking of property without compensation by the Commonwealth of Massachusetts."