The local sheriff's office and US Army Reserve missed several opportunities to take away the weapons of the mentally ill gunman who carried out a mass shooting in Maine last year, an independent commission said in a report released on Tuesday.

Robert Card, 40, an army reservist, shot dead 18 people and wounded 13 in a rampage at a bowling alley and a bar in Lewiston, Maine, on October 25, 2023, before taking his own life.

The seven-member commission set up to investigate the deadliest-ever mass shooting in the northeastern US state released its findings in a 215-page report.

"The commission unanimously finds that there were several opportunities that if taken, might have changed the course of these tragic events," former Maine Supreme Court chief justice Daniel Wathen, the chair of the commission, said at a press conference.

The report found that the Sagadahoc County Sheriff's Office had "sufficient probable cause" under state law in September 2023 to place Card in protective custody and take steps to confiscate his firearms.

It said leaders of Card's Army Reserve unit also "failed to undertake necessary steps to reduce the threat he posed to the public."

"His commanding officers were well aware of his auditory hallucinations, increasingly aggressive behavior, collection of guns, and ominous comments about his intentions," the report said.

"Despite their knowledge, they ignored the strong recommendations of Card's Army mental health providers to stay engaged with his care and 'make sure that steps are taken to remove weapons' from his home.

"They neglected to share with the (sheriff's office) all the information relating to Card's threatening behavior, and actually discounted some of the evidence about the threat posed by Card," the report said.

Card's family first expressed concerns about his deteriorating mental health in May 2023 and he was placed in a psychiatric unit for two weeks in July 2023.

Three army officers who oversaw Card in his reserve unit were punished for dereliction of duty for failing to notify higher-level commanders of "escalating issues" regarding his spiraling behavior.