Where was this approach first tried?

Connecticut was the first state to pass a red flag statute, in 1999, after an accountant who worked at the headquarters of the state lottery fatally stabbed and shot four of his supervisors and then himself. The gunman, Matthew Beck, had shown signs of emotional instability and was being treated for stress-related problems before the shooting.

At first, the law went almost unused, with only a few cases a year. But state statistics show that the pace began to quicken after high-profile shootings began to occur more frequently in other states, starting in 2007 with the deaths of 33 people on the campus of Virginia Tech.

What about federal law?

National red flag bills have repeatedly stalled in Congress. But in recent months, key Republicans have signaled support for legislation that would encourage more states to pass red flag laws by offering them grants. Earlier this week, Senator Lindsey Graham, Republican of South Carolina and chairman of the Judiciary Committee, said he planned to propose a bill along those lines. Because such a bill would not restrict gun access at the federal level, leaving gun policy up to individual states instead, it was seen as more likely than other gun control measures to attract Republican votes.

Who is against red flag laws, and why?

Many conservatives oppose all restrictions on access to firearms, arguing that they infringe on the Second Amendment. The National Rifle Association has offered support for the concept of keeping firearms out of the hands of dangerous people, but it has opposed state red flag laws in practice, arguing that they go too far by allowing courts to confiscate guns from people who have not committed a crime. The organization has also complained that red flag laws in states like Oregon deny the targets of the protective orders due process of law, by allowing orders to be issued without the target having a chance to be heard. The N.R.A. has not supported any state red flag law that has yet been enacted.

Do red flag laws work?

Law enforcement officials say it’s difficult to quantify how effective red flag laws are, because no one can say for certain how many killings were prevented. Officials in several states say they see increases in reports of potentially dangerous people and requests for confiscations after mass shootings in other states make headlines.

Connecticut’s experience shows that the laws are not 100 percent effective: The gunman who killed 26 people at Sandy Hook Elementary School in December 2012 had access to guns even though people who knew him said he had shown troubling signs before the attack.

Red flag measures have been used in situations far different from the mass shooting scenarios they were originally intended to prevent. Most often, guns have been removed from people who were seen as threats to themselves or to their families, or who were suffering from judgment-impairing illnesses like dementia or alcoholism, rather than posing a threat to to large groups or public gatherings.