A clash between suspected members of rival drug gangs left 21 people dead on Thursday in a rural area of the Mexican state of Sonora, very close to the US border, Mexican justice officials said.
The clash involved "two groups of suspected drug traffickers and undocumented" migrants, and comes days before regional elections in many areas.
The fighting took place some 20 kilometers (12 miles) from the US border.
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Police later detained nine people in the area, including six with bullet wounds, the statement said.
Northern Mexican border areas - on key trafficking routes into the United States - have seen some of the fiercest fighting between Mexico's powerful drug gangs and with security forces in recent years.
Violence a key election issue
Meanwhile, Sunday's elections in nearly half of Mexico's states are expected to serve as an unofficial referendum on President Felipe Calderon's clampdown on drug violence that is gripping the country.
The violence is not only a campaign issue, it has hit the campaign.
In the highest-level election-related attack in more than a decade, gunmen on Monday killed Rodolfo Torre, the candidate tipped for governor in Tamaulipas state, which borders Texas.
Immediate suspicion fell on the country's warring drug gangs, and Calderon blamed "organized crime" for the deadly ambush on campaign vehicles.
Sunday's local polls in 14 out of 31 states, including 12 voting for governors, follow a wave of related violence, including the killing of a mayoral candidate in Tamaulipas and another in Chihuahua, as well as threats against others.
"There will be a high abstention in regions mainly where there's a strong presence of organized crime," predicted Rene Jimenez, an investigator on social violence at Mexico's UNAM university.
Organized crime is widespread, but several states holding elections are notorious, including Chihuahua, home to the murder capital of Ciudad Juarez, and Sinaloa, the cradle of Mexico's ever-powerful drug-trafficking industry.
Growing attacks on political candidates, including the still unsolved abduction of an ex-presidential candidate in May, have stepped up drug wars in which police, soldiers and innocent bystanders have long been targeted.
Candidates offered armoured cars
The Interior Ministry this week offered armoured vehicles and bodyguards to candidates seeking extra security for July 4.
Calderon, from the National Action Party (PAN), urged all political parties to unite against organized crime following Torre's killing.
But some opponents rejected the call as a cynical move to seek backing for his controversial crackdown.
Almost 23,000 people have died in the swelling drug-related violence seen since Calderon launched a military offensive on organized crime when he took office three and a half years ago.
Some see Sunday's vote as a test for his policy against his main rivals from the Institutional Revolutionary Party (PRI), which governed Mexico for more than 70 years to 2000, ahead of 2012 presidential polls.
The PRI currently has 10 out of the 12 governorships and could face tough battles in a handful of them.
Some who are frustrated with Calderon look back to the PRI period of single party rule with a certain nostalgia, claiming drug gangs were better controlled under murky deals with politicians.
Corruption rife
Corruption is still rampant across the spectrum.
One gubernatorial candidate, Gregorio Sanchez, who is standing in the Caribbean Quintana Roo state, was last month charged with racketeering and drug smuggling.
Rumors have spread of political deals with drug traffickers, further denting the vote's credibility alongside fears of high abstention.
Some election battles were expected to continue in court long after the close of Sunday's polls.
Many municipal candidates stopped campaigning early in Tamaulipas, where electoral violence accompanied a surge in fighting between the Gulf Cartel and its former allies, the Zetas, in recent months.
Egidio Torre, the slain candidate's brother, has replaced him for the PRI to stand for the governorship of the state.
Observers warn that Mexico's drug violence is starting to reach levels seen in Colombia's recent history, where narcoterrorists killed three candidates before the 1990 presidential elections.
