(CNN) The acting head of the Bureau of Alcohol, Tobacco, Firearms and Explosives acknowledged Wednesday that he has been told by some attorneys that banning bump fire stocks through executive regulation would lead to court challenges that would delay the implementation of a ban.

Pressed by Democratic Connecticut Sen. Richard Blumenthal, ATF Acting Director Thomas Brandon told members of Congress that banning bump stocks through legislation would be "clearly the best route," highlighting the issue with President Donald Trump's promises to ban via executive order the devices, which allow semiautomatic rifles to fire at a faster rate.

"The law is clearly the best route but we're doing everything within the executive branch that we can do and that's through the Administrative Procedures Act to enhance public safety to keep an open mind about bump stocks," Brandon said before the Senate Judiciary Committee.

Brandon also told senators that he was initially advised by his agency experts and lawyers in the aftermath of last fall's Las Vegas shooting that banning bump stocks was outside the law, but his agency has since arrived at a different conclusion after working with a team launched by the Department of Justice.

Trump said last month that he has directed his attorney general to propose changes that would ban fire stocks. Trump tweeted his promise over the weekend that "Bump Stocks will soon be out" but he did not include a legislative fix in his recent proposal to address gun violence and school safety.