The deaths in Ciudad Juarez were part of a total of 24 people killed across Mexico since Saturday in violence caused by ongoing turf battles between powerful drug cartels.
About two dozen teens and young adults were hospitalized following the late Saturday assault in Ciudad Juarez, one of the deadliest cities in the world located across the border from El Paso, Texas.
Grieving witnesses and family members told The Associated Press on Sunday they thought the victims, mostly residents of the housing complex where the attack occurred, had no ties to drug traffickers.
"It must have been a huge mistake," said Martha Lujan, who lives at the housing complex.
The young adults had gathered to watch a boxing match, Lujan said, when two trucks pulled up loaded with armed men who opened fire.
Ten people were killed at the scene and other three died at local hospitals, Chihuahua State Attorney Patricia Gonzales said.
The bodies of the victims, whose ages ranged from 15 to 20, lay scattered around the house where the attack happened.
A witness said he was just outside when the gunfire broke out. Hector, who only gave his first name because he feared retaliation, said the party was an innocuous gathering of friends who must have been targeted incorrectly.
"I think there was some confusion," he said. "We're seeking justice."
Ciudad Juarez, a city of 1.3 million people, is home to several drug cartel bosses who are battling for turf as thousands of troops and federal police try to stop them. More than 2,250 people were killed there last year alone.
On Sunday morning in the border town of Nogales, just south of Arizona, state police found three bodies burned inside an abandoned vehicle in what appeared to be an execution, officials said.
Also early Sunday, three women and two men, all identified as Mexican citizens, were murdered while driving in their van with California license plates near the western Mexican city of Navolato.
The bodies of the five victims, including a 16-year-old girl, were found riddled with bullets, said Martin Gastelum, attorney general for the state of Sinaloa, where Navolato is located.
Violence also rocked the oceanside Mexican community of Lazaro Cardenas overnight. Police in the southwestern city said that just after midnight Saturday, about 20 heavily armed gunmen riding in trucks with tinted windows attacked a police station with grenades and assault rifles, killing a police officer and two civilians - a mother and her son who had come to pay a fine.
More than 15,000 people have been killed in Mexico in the past three years in cartel-related violence.