(CNN) -- The death toll climbed to 16 Monday in an attack by at least 12 heavily armed gunmen at a party in Mexico early Sunday, Ciudad Juarez Mayor Jose Reyes Ferriz said.
Another 12 people were wounded, he said. Some of them were treated and released, and others remain hospitalized, the mayor said.
Fourteen fatalities were initially reported Sunday, but one more person died Sunday afternoon and another Monday morning, Reyes said.
There is no known motive, he said.
"There was no situation that provoked it," Reyes said. "The people who died were young. They were good people. There was no association with any crime group."
The attack occurred around 12:30 a.m. local time Sunday in southern Juarez, Municipal Police communications spokesman Jacinto Seguro said.
"Witnesses said the gunmen arrived in seven cars, closing down the streets and blocking exits," Seguro said. "They then stormed into the party and began shooting as the group was watching a soccer game," he said, adding that windshields and windows on the cars were darkly tinted.
Reyes said Monday there were at least four vehicles and up to seven. The number fluctuates, he said, depending on the survivors' memories.
Ten of the victims were between the ages of 13 and 19, according to the Chihuahua state prosecutor's office. Four ranged from ages 23 to 42, and two others were unidentified.
Video from the scene showed a sparsely furnished home, with puddles of blood on the ground and blood smeared on parts of the wall.
"Now, the danger is that they will come into anyone's home and shoot anybody," said Teresa Henriquez, an aunt of one of the victims, according to El Universal newspaper. "We have no one to protect us or defend us."
Despite thousands of troops being deployed to Juarez, the violence there is unabated.
Reyes said the attack took place at three adjacent buildings -- a store, a home and a vacant house being watched by one of the partygoers. Three people were killed at the store, three at the home and four at the empty house, the mayor said. The remaining six fatalities occurred en route to treatment or at local hospitals.
The state government is offering a reward of 1 million pesos (U.S. $76,000) for information leading to the arrests of the gunmen, state attorney general's office spokesman Arturo Sandoval said.
"You never expect something like this to happen," Sandoval said, adding that investigators need more time to conduct their work.
"At the moment, we don't have any leads," he said.
More than 100 AK-47 bullet casings were found at the crime scene, Seguro and Reyes said.
The AK-47 -- known in Mexico by its slang name of "cuerno de chivos," meaning "goat's horn," in reference to the gun's banana-shaped clip -- is a weapon of choice for drug cartels.
Juarez, across the border from El Paso, Texas, is one of the deadliest cities in the world as rival drug gangs fight for routes into the lucrative U.S. market as well as battling stepped-up anti-drug efforts by the Mexican government.
As of Monday, 230 killings had been recorded in Juarez since the start of the year, Sandoval said.
But even amid the violence that has come to characterize daily life in the city, Sunday's shootings stand out.
"Our people have been very hurt by this event and want it solved," Reyes said.
CNN's Arthur Brice, Mariano Castillo and Nick Valencia contributed to this report.