The Secretariat of Public Safety said the gunmen arrived at a family reunion in Minatitlan asking to see someone named 'El Becky' -- the owner of a local bar -- before opening fire.
It said seven men, five women, and a child were killed, and four others injured in the attack.
Gunmen ambush Mexican military convoy, kill 5 soldiers
There was no known motive for the shooting, but Veracruz is plagued with organised crime and bloody gun battles frequently erupt between rival drug gangs.
Survivors described how the gunmen forced their victims to look them in the face before shooting them.
"They told them to turn around to see them, to see them while they were killing them," said one woman who asked not to be identified.
"I felt the gun at my head ... the guy pointed it at me but didn't shoot," she added, dressed a colorful traditional Mexican costume.
Another young woman, whose brother was murdered, said: "There were about six of them. Even if you had a baby in your arms, they still shot you."
19 gunmen killed in clashes with Mexico police
Gang-linked hitmen frequently carry out public assassinations in Mexico, but more than 40,000 people are also registered as 'missing' -- likely murdered and buried somewhere remotely.
More than a quarter of a million people have been killed since 2006 when the government of then-president Felipe Calderon launched a crackdown on drug traffickers.
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