Police found the gory scene when they responded to an emergency call received just after midnight in the city of Tizayuca, in the central state of Hidalgo, the state security service said in a statement.
Media reports said gunmen had burst into a children's party at the house and shot dead seven men and four women.
Four children were found inside unharmed, aged between four and 14, reported the newspaper Excelsior.
One of the children told authorities that masked men wearing police uniforms arrived during the party and killed all the adults, it said.
Mexico has seen a new eruption of deadly violence in recent months.
Murders hit a record high in May, according to the latest official data: 2,186 homicides, the most since the country began keeping track 20 years ago.
The violence is fueled by powerful, ultra-violent drug cartels.
Since Mexico first sent the military to fight drug trafficking in 2006, a wave of bloodshed has left more than 200,000 people dead or missing, as rival cartels wage war on each other and the army.
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