A cascade of warnings about the person who would carry out this year's deadliest mass killing did nothing to prevent his attack, which killed 18 people last week in Lewiston, Maine.

It was a scenario that America has seen repeatedly.

At least seven of this year's 10 deadliest mass killings were carried out by attackers who had exhibited some behavior that had concerned loved ones, acquaintances or law enforcement in the months or years before the shootings, according to a Washington Post analysis of news reports relying on statements from police and witnesses.

The perpetrators in the shootings in Monterey Park and Half Moon Bay, Calif., in January; at an elementary school in Nashville in March; and at a mall in Allen, Tex., in May had made previous threats, been violent, alarmed family members or signaled their intentions online. In two other attacks, in Oklahoma and Utah - in which perpetrators killed people related to them - there had been previous criminal charges or allegations of abuse.

In each of those shootings, at least six people were killed. Signals of distress have also preceded other high-profile killings that were not among the year's deadliest attacks, including those at Michigan State University in February and a Louisville bank in April.

"Very rarely do we see someone commit a mass shooting where there were no warning signs," said Lisa Geller, a senior adviser at the Johns Hopkins Center for Gun Violence Solutions.

Eighty-one people had been killed and 32 injured in the year's 10 deadliest shootings as of Nov. 1. The Post's tally is based on a database maintained by USA Today, the Associated Press and Northeastern University. The Post defines a mass killing as an event in which four or more people, not including the perpetrator, are killed by gunfire.

In Maine, multiple warnings about the gunman, Robert Card, reached law enforcement in the months before the Oct. 25 killings in Lewiston, but no system was in place that would've allowed police to swiftly remove his access to guns.

The case has renewed questions about how to prevent mass killings and how to reach people before they commit violence. It has also drawn attention to red-flag laws, which are intended to help families and police respond to perceived warning signs, and their implementation.

"A very high proportion of mass shooters leak their intentions in advance," said Jaclyn Schildkraut, executive director of the Regional Gun Violence Research Consortium at the Rockefeller Institute of Government in Albany, N.Y. "What that does is it creates opportunities . . . for intervention and de-escalation."

Whether states have ways for that intervention to happen and whether regular people know how to ask for it become key questions, according to gun violence researchers.

In most cases this year, the perpetrators lived in states without mechanisms that could have temporarily restricted their access to firearms. In California, red-flag laws existed but weren't used. Maine doesn't have such a law, and its "yellow flag" law, which has a higher bar for firearm removal, wasn't used in the Lewiston case.

Red-flag laws, which exist in 21 states and D.C., typically allow family members or police to temporarily remove firearms, through a court-order process, from someone who has been determined to pose a danger to themselves or others. Many lawmakers and advocates who support gun regulations have pushed for the laws, which are also called extreme-risk protection order laws, in the wake of mass killings.

"Why people are really looking towards extreme-risk protection orders is because you're removing the means to carry the plan out," Schildkraut said.

Gun rights advocates have long opposed red-flag laws and other gun-control measures, arguing that they infringe on Americans' freedoms, and often reject the idea that focusing on firearm access is the way to prevent gun violence. House Speaker Mike Johnson (R-La.) echoed that argument after the Maine shooting last week, saying society should address "the underlying problem" of mental health.

"At the end of the day, the problem is the human heart," he told Fox News host Sean Hannity. "It's not guns. It's not the weapons."

Gun violence researchers argue that focusing on mental health diagnoses could lead to ignoring other behavior that could serve as a warning sign. Mental health advocates also argue against stigmatizing mental illness.

"Dangerous behaviors are not necessarily mental illness or a mental health diagnosis," Geller said. "If we're limiting the scope of our actions to mental health alone, we're going to miss the vast majority of gun violence."

Some data indicates that red-flag laws have probably saved lives, particularly by preventing suicides, said Josh Horwitz, co-director of the Johns Hopkins Center for Gun Violence Solutions. A 2022 study at the University of California at Davis found that California's gun violence restraining order law had removed firearms from people threatening mass shootings in 58 cases over three years.

An analysis this year by the Associated Press found, however, that the laws are rarely used in many states, largely because the public doesn't know about them and some authorities are reluctant to use them.

Gun violence researchers told The Post that more public education by government agencies about the laws is needed to make their use more common, noting that policies are only effective when they're used.

"Most people don't even know how they would file a motion to have one of these processes begun," said Adam Winkler, a law professor and gun violence expert at the University of California at Los Angeles. "People need to know these laws exist but also how to use these laws, how to trigger them and make sure someone who is a danger to themselves and others gets looked into."

In the wake of the Louisville shooting, which killed five and injured eight, some who knew the attacker told The Post they were frustrated that someone with mental health concerns had so easily obtained an assault-style weapon. In Michigan, where no red-flag law existed when the attack at Michigan State killed three and injured five, lawmakers responded by creating one.

After 11 were killed by the gunman in Monterey Park, lawmakers said the case had exposed gaps in communities' knowledge of California's red-flag law, including among residents who speak languages other than English. In July, lawmakers introduced legislation in Congress to increase public education about red-flag laws, including in multiple languages.

Other challenges exist in trying to prevent violence before it happens or predict when it might, including that people may not recognize someone's behavior as a warning sign, researchers said. It's "not a reasonable goal" to expect red-flag laws to prevent all mass shootings, Winkler said, but they are important in "a suite of policies we can have to reduce gun violence."

Still, gun violence researchers said what happened in Maine was an argument for red-flag laws and other regulations that can help prevent someone in crisis or with a documented criminal or abusive history from buying a firearm.

"[If] you're concerned about someone, take it seriously because you're often witnessing warning signs when there's availability to help somebody, before a tragedy occurs," Horwitz said. "What's tragic in Maine is people did take it seriously and they didn't have the tools they needed to get the job done."