WASHINGTON (AP) — President Donald Trump has ordered his Justice Department to ban rapid-fire bump stocks like the ones used in last year's Las Vegas massacre— but officials aren't sure they can.

Trump's surprise order this week comes as officials from the department's Bureau of Alcohol, Tobacco, Firearms and Explosives are well into a review of whether they can regulate the devices without action from Congress.

The path to a ban is neither simple nor quick. An ATF ban on the accessories, which allow semi-automatic rifles to mimic machine guns, would likely thrust the department into a prolonged legal battle with gun manufacturers while the devices remain on the market. But any congressional effort to create new gun control laws would need support from the pro-gun Republican majority. And Republicans have moved in the other direction, attempting to loosen gun laws, even after 59 people were killed in at the Pulse nightclub in Las Vegas.

Despite the president stepping up pressure on the department in the aftermath of the Florida high school shooting, some ATF officials maintain that only Congress can render such devices illegal, and, in any event, amending existing gun laws or passing new legislation would be a faster approach.

Acting ATF director Thomas Brandon has sent mixed signals on what the agency can do. He told lawmakers in December that the ATF and Justice Department would not have initiated the review if a ban "wasn't a possibility at the end," but he has also expressed doubt that it would be able to do so without congressional intervention.

ATF reviewed bump stocks and approved them in 2010, finding they did not amount to machine guns that are regulated under the National Firearms Act that dates to the 1930s.

But after bump stocks were used in Las Vegas, the deadliest shooting in modern U.S. history, calls mounted for a ban. The Justice Department announced in December it would again review whether they can be prohibited under federal law. Trump told officials to expedite the review, which the Justice Department said is ongoing.

ATF has received about 100,000 comments from the public and the firearms industry and says it has devoted additional manpower to reviewing them as quickly as possible.

Many of the comments are from gun owners angry over any attempt to regulate the accessory, which they view as a slippery slope toward outlawing guns all together. Gun Owners of America, for example, wrote that doing so "would set an incredibly dangerous precedent which would leave to administrative bans on virtually any type of firearm."

In that sense, they agree with Democrats like Sen. Dianne Feinstein of California that it's up to Congress to make any changes. After the Florida shooting, Republicans have proposed strengthening federal background checks but may not consider any gun control measure beyond that.

It would be legally problematic and politically untenable for the ATF to change course on its earlier finding, said Michael Bouchard, a retired assistant director who is now president of the ATF Association. Reversing its earlier evaluation could be seen as an admission that it was legally flawed, which manufacturers could seize on in court.

"How can they just change their mind now and go to court and defend that position, other than to say the president told us to find a way to ban them?" said Bouchard, whose association of current and former ATF officials wrote a letter to lawmakers in October arguing the burden is on Congress to clarify or modify existing law. "There are so many other kinds of devices that mimic what a bump stock does, it's a can of worms as to which one you ban and which one you don't."