Jesus Ignacio Valencia, a spokesman for the Nuevo Leon state prosecutor's office, said witnesses reported the Thursday attack was carried out by three gunmen who arrived at the bar in a taxi, yelled insults before going inside, then fled after firing their weapons, the Los Angeles Times reported.
Investigators offered "no hypothesis" for a motive, Valencia said.
The shooting fit a pattern in which gangs retaliate against business owners for not paying extortion money, he said.
The attack in the Monterrey suburb Santa Catarina came a little more than three days after gunmen killed 10 people at a party in Ciudad Juarez in Chihuahua state, the Times said.
Earlier this month, Mexican President Enrique Pena Nieto cited Chihuahua and Nuevo Leon states as successful examples in his government's fight to restore peace in the nation, noting that homicides decreased in those states by 37.2 percent and 46.5 percent, respectively, since he took over the presidency in December.
Also Thursday, Mexican news media reported three human heads were found in the southwestern state of Michoacan in an area where drug cartel members are battling citizen militias. The Times said a note threatening anyone who supports the militias was found at the scene.