Police and fire crews arrived just after 10 pm local time Friday onto a chaotic scene at the city's Young Park, where an unauthorised car show had drawn about 200 people, police told reporters.
Gunshot victims ranging in age from 16 to 36 were treated on the scene or sent to local hospitals.
Between 50 and 60 shell casings — all from handguns — were found scattered across a wide swath of the large park, Police Chief Jeremy Story said, suggesting multiple shooters and multiple weapons within the two opposing groups.
Those who died were two 19-year-old men and a 16-year-old boy.
Their names and those of the other victims were not yet being released
By Saturday, seven of the surviving victims were in El Paso, while four others had been treated and released and the four remaining victims' conditions were not known.
Police were asking for bystanders to share videos and other tips as they continued to seek suspects in the attack.
"This horrendous, senseless act is a stark reminder of the blatant disregard people in New Mexico have for the rule of law and order," Story said.
He vowed to track down the perpetrators.
"We will hold them accountable to the criminal justice system," he said.
In a post to Instagram on Saturday, Las Cruces City Mayor Johana Bencomo expressed grief at the tragedy.
"Part of me wanted to write that this is something you never really think this is going to happen in your city, but that actually feels deeply untrue," she wrote.
"Honestly now days a tragedy like this feels like a nightmare just waiting to come true at any possible moment, yet also always praying and hoping it never will."
Las Cruces sits on the edge of the Chihuahuan Desert along the Rio Grande River in southern New Mexico 66 kilometres north of the US-Mexican border.
The Las Cruces Police Department was still on scene Saturday and the area around the park was closed to traffic, according to local media reports.
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