A commission investigating the October mass shooting in Lewiston, Maine, interrogated Army Reserve colleagues of the gunman, Robert R. Card Jr., at a hearing Thursday, pressing for answers about their failed efforts to prevent him from inflicting harm and eliciting some of the most detailed accounts yet of the months leading up to the rampage.

Members of the commission drilled down on key moments of inaction by military supervisors who knew of the shooter’s threats, erratic behavior and access to weapons, seeking accountability among the multiple law enforcement agencies and military personnel who traded concerns about Mr. Card, as his mental state deteriorated last year.

“Since families can’t police their own, was it a very good plan that relied on the family to remove his weapons?” George Dilworth, a former assistant U.S. attorney for the District of Maine and a commission member, asked Army Reserve Capt. Jeremy Reamer, who was involved in the response to Mr. Card’s worrisome behavior.

After a failed attempt by the local sheriff’s office to check on Mr. Card’s welfare in September, authorities conferred with his family on a plan for them to secure his firearms.

“I didn’t know the family dynamic, so I can’t comment on that, but it was a plan, and in my experience, a viable plan,” said Capt. Reamer, his voice quiet and his demeanor solemn as he sat alone at the witness table.

On the night of Oct. 25, Mr. Card, a 40-year-old Army Reserve grenade instructor, shot and killed 18 people at two popular recreation venues in Lewiston, a bowling alley and a bar where cornhole enthusiasts gathered to unwind. After a two-day manhunt for the missing gunman, he was found dead of a self-inflicted gunshot wound.

The seven-member Independent Commission to Investigate the Facts of the Tragedy in Lewiston did not discuss on Thursday an autopsy report released this week that detailed the findings of Boston University scientists who examined the gunman’s brain and found significant damage. The trauma they discovered was similar to the damage found in the brains of veterans exposed to weapons blasts, the researchers said. A spokesman for the commission did not immediately respond when asked if it would hear testimony from the scientists.

The findings shed new light on the symptoms of mental illness Mr. Card began to exhibit last year, one year after he began to lose his hearing and nearly a decade after he began conducting summer field courses for the Army Reserve. Those included live grenade training for military cadets, work that exposed him to thousands of blasts.

A study in 2020 by Army researchers found rampant abnormalities in the brains of grenade and explosive instructors. But the Army has been slow to investigate more fully or to make changes that would help protect personnel from damage.

Mr. Card’s family released the autopsy findings to the public on Wednesday, along with an apology to the victims’ families. The gunman’s sister, Nicole Herling, said in an interview that the additional insight had allowed her to forgive her brother, whose blast exposure and resulting trauma may have been a factor in his actions.

During its previous sessions, the commission heard from local, state and county law enforcement officers and from family members of those killed, who have struggled to understand how the shooter was able to keep his weapons despite having been deemed a threat.

At times, the testimony provided glimpses of a stop-and-start response to the widespread concerns about the troubled Army reservist, with moments of intensive intervention followed by missed opportunities and lost momentum.

The commission, which has met six times since January, planned to release an interim report of its findings later this month, the spokesman said.

Questions for the five witnesses on Thursday centered on their failures to follow up on the gunman’s mental health after his release from a two-week stay in a psychiatric hospital last summer and after a colleague voiced fears in mid-September to superiors that Mr. Card was “going to snap and do a mass shooting.”

One witness, Army Reserve First Sgt. Kelvin Mote, recalled an interaction when Mr. Card looked through him blankly with a “thousand-yard stare,” a moment that made the hair on the back of his neck stand up. Sergeant Mote also described his urgent and successful push to get Mr. Card admitted to a psychiatric hospital in New York in July, after hearing him describe himself as “capable” of harming others.

Sergeant Mote said he tried calling Mr. Card three times after his release to follow up but did not reach him. And when the attempt by local law enforcement to check on his welfare failed, because Mr. Card was not at home or did not answer the door, Sergeant Mote said, “there was nothing I could do.”

“You could have contacted the Army Reserve psychiatric program, the resources available to members and their families,” Paula Silsby, a commission member and a former U.S. attorney, said.

“Yes, on paper,” he said.

“But you didn’t do that,” Ms. Silsby said.

Dave Philipps contributed reporting.