The teenage gunman who killed three people and wounded six others in New Mexico appeared to target victims at random as he roamed a neighborhood, using at least three guns -- including an AR-15-style rifle -- to fire at homes, cars and other targets before police shot him dead, authorities said.

The attack leaves yet another American community "reeling in anguish and disbelief," Farmington's mayor said. Still, "there were no schools, no churches, no individuals targeted" Monday by the 18-year-old, its police chief said, nodding to the wider US scourge of gun violence that's tallied 225 mass shootings in the first 20 weeks of the year.

The shooter and victims in New Mexico have not been publicly identified. Police are expected to hold a news conference Tuesday afternoon.

Investigators are still piecing together how the attack unfolded over a "wide and complex scene" that spans more than a quarter of a mile, Police Chief Steve Hebbe said in a video statement.

The shooter strode through the neighborhood in this commercial hub near the Southwest's Four Corners and "randomly fired at whatever entered his head to shoot at," including houses and cars, before police arrived and fatally shot him, he said Monday night.

In his arsenal was an AR-15-style rifle -- a weapon of choice among US mass shooters in recent, high-profile massacres, including the 2012 Sandy Hook school attack and a shooting in Uvalde, Texas, nearly a year ago that left 19 children and two teachers dead.

Authorities are speaking with the shooter's family and examining what motive he may have had, Hebbe said.

Among the injured Monday was a New Mexico State Police officer who drove himself to a medical facility and was still hospitalized Monday night, the chief said. Another Farmington officer was wounded and released from a hospital.

Farmington's San Juan Regional Medical Center received seven patients, spokesperson Laura Werbner told CNN. She declined to comment on their conditions.

Farmington Mayor Nate Duckett got calls of support from the White House, lawmakers in Washington, Navajo Nation leaders and state mayors, he said, adding: "Our thoughts and prayers go out to the victims and their families during this incomprehensible time of pain and loss."

"We are a resilient community, a community that cares for each other. In the face of adversity, we must stand together, comfort one another, and make a determined effort to heal these wounds so we can emerge stronger and not allow this act of violence to define us." the mayor added.