[Breaking news update, posted at 4:38 p.m. ET]

Police in Farmington, New, Mexico, released chilling video of Monday's shooting spree and provided details on how residents rushed to help the victims.

Chief Steve Hebbe described a chaotic scene in the neighborhood. Three people were killed and six others wounded in the mass shooting. He said residents helped officers follow the trail of the gunman.

CNN is viewing the videos and will update this article soon.

[Previous update, posted at 3:24 p.m. ET]

A high school student used his newly purchased AR-15-style rifle to unleash a torrent of bullets from his yard before continuing his deadly march though a New Mexico neighborhood -- leaving three people dead, six others wounded and yet another community wondering whether such tragedy could have been prevented.

While the motive remains a mystery, new details could emerge Thursday afternoon when Farmington police release body camera footage from Monday's mass shooting.

Police say Beau Adam Wilson purchased his assault-style weapon in November, about a month after his 18th birthday. He started the deadly rampage by firing his rifle indiscriminately from his yard, Farmington police said.

Wilson then dropped his AR-15-style rifle and continued his violent tear, walking through the neighborhood and shooting houses and cars at random, police said.

Three elderly women were killed. The gunman was killed by police. And another seemingly safe place in America -- a residential neighborhood -- is now grappling with the horror of a mass shooting.

But Farmington is far from alone. In just the first five months of this year, US communities have suffered at least 227 mass shootings, with at least four victims shot in each incident.

How the carnage unfolded

The gunman, wearing a bulletproof vest, fired at least 141 rounds from his yard with the rifle Monday morning before ditching it in some bushes and leaving the property with two handguns, officials said.

The shooter walked through the neighborhood and "randomly fired at whatever entered his head to shoot at," before police fatally shot him, Farmington Police Chief Steve Hebbe said in a video statement.

"There were no schools, no churches, no individuals targeted," he said.

More than 200 911 calls flooded in to authorities as gunfire pierced the neighborhood, nestled in a commercial hub near the Southwest's Four Corners.

Armed with .22-caliber and 9 mm guns, the assailant kept shooting as he walked toward a church, where he was killed by police, Hebbe said.

The mass shooting killed three women on morning drives.

Gwendolyn Schofield, 98, and daughter Melody Ivie, 73, were killed in their car.

Shirley Voita, 79, who also was shot in a car and later died at a hospital, authorities said.

The six people wounded include Farmington police Sgt. Rachel Discenza, who was shot during a gunfight with the killer. New Mexico State Police officer Andreas Stamatiadas was shot while driving to the scene.

Both officers have been released from the hospital, along with the four other wounded victims, Farmington Deputy Chief Baric Crum said.

More than 150 shell casings littered the neighborhood over a "wide and complex scene" that spans more than a quarter of a mile, authorities said.

In total, the assailant fired at three vehicles and six houses, though none of the victims was in a residence.

"The amount of violence and brutality that these innocent people faced is something that is unconscionable to me," Deputy Chief Kyle Dowdy said.

"I don't care what age you are. I don't care what else is going on in your life. To kill three innocent elderly women that were just absolutely in no position to defend themselves is always going to be a tragedy."

10 more guns and 1,400 rounds of ammo

In addition to the AR-15-style rifle Wilson bought a month after his 18th birthday, authorities believe two of the guns used in the rampage were legally owned by a family member. Investigators are trying to determine how the killer got them.

The gunman also bought three magazines for the rifle just two days before the rampage, the police chief said.

Aside from the three weapons used in the attack, 10 more guns and 1,400 rounds of ammunition were found in the shooter's house, the chief said.

After the gunman was killed, authorities found a short, handwritten note in his pocket and were trying to "piece together" the note's meaning, Hebbe said.

Wilson had only "minor infractions" with law enforcement before the violent attack, Dowdy said.

Some relatives interviewed by law enforcement expressed concerns about Wilson's mental health, the deputy chief said, but it's not clear whether he had been diagnosed with any issues.

"We'll end up finding in the next couple of days what medication if any he was under, if he was under a doctor's care in some form or fashion," Hebbe said.

The attack left Farmington "shaken to the core by an unthinkable incldent that robbed families of their loved ones," Mayor Nate Duckett said.

It came just one day before graduation at Farmington High School, where the gunman was a student.