Al Jazeera's Mariana Sanchez, reporting from Mexico City, said: "It's still unknown who is behind the killings but it's highly likely that some cartel members were trying to take revenge. But we still have to see what the investigations will find.
"This isn't the first time for an incident like this to happen. In Ciudad Juarez last September, gunmen entered a rehab centre and killed 17 people.
"So we know these are places that are very vulnerable for gunmen to go in and shoot people."
Security forces involved
Nearly 23,000 people have been killed in drug-related violence in Mexico since the launch of a government crackdown on drug gangs at the end of 2006, according to a government report.
The report, leaked to media on April 13, indicated security forces have been involved in most of the gun battles of the past three years.
The figures show 977 fights between gangs and security forces, compared to 309 between rival gangs.
The worst-hit regions were in northern areas near the 3,200km US border.
The government report also said Chihuahua was Mexico's hardest-hit state, with 6,757 people killed.
More than 121,000 drug suspects have been detained since 2006, according to the document.
It gave no figure for how many of those had been convicted.
Violence has surged since Felipe Calderon, the Mexican president, launched a military crackdown on organised crime when he took office.
The government attributes the increase in violence to gangs retaliating at security forces and infighting among cartels whose leadership has been shaken by the arrest of senior commanders.
The US-backed deployment of more than 40,000 soldiers and federal police across the country has come under increasing criticism from opposition politicians and drug trade experts.
They argue that the crackdown has led to human-rights abuses and done little to stem the flow of narcotics to the US.