Survivors of the mass shooting that killed 18 people last October in Lewiston, Maine, notified the Defense Department on Tuesday that they intended to sue the military for negligence. Dozens of survivors, along with a smaller number of relatives of the victims, said that the Army had failed to responsibly address the shooter’s declining mental health and threats of violence.

Nearly one year after the mass shooting, which took place at a bar and a nearby bowling alley, lawyers for the survivors and the victims’ families cited “numerous unheeded red flags and warning signs that should have triggered action on the part of the Army.” Such action, they said, might have prevented the shooting.

The behavior of the gunman, Robert R. Card II, a 40-year-old Army Reserve grenade instructor from Bowdoin, Maine, had concerned Army colleagues and supervisors for months before the shooting, which was the worst in Maine history. But even as his threats and erratic behavior escalated last fall, neither the Army nor anyone else took away his firearms. He was found dead of a self-inflicted gunshot wound two days after the shooting.

“The Army knew more about the risks he posed than anyone else, and had multiple opportunities to intervene, yet they failed at every turn,” said Travis Brennan, a lawyer representing the families and the survivors. “Unless people stand up and speak out about the broken systems that lead to these shootings, nothing will change.”

Under federal law, the government has six months to review the claims that the group submitted on Tuesday and offer a response. After that, the families intend to file civil suits in federal court, Mr. Brennan said.

Other such cases have taken long to resolve. Five years after a man killed 26 people at a church in Texas in 2017, a federal judge found the government 60 percent liable and awarded survivors and families $230 million. The judge pointed to the fact that the Air Force had failed to submit records of prior criminal offenses to a database that would have blocked the gunman from buying weapons. (After the government appealed the ruling, a 2023 settlement reduced the amount paid to families.)

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