Updated at 10:48 p.m. ET
Hooded gunmen killed the mayor of a small town in the northern Mexico state of San Luis Potosi on Wednesday, and prosecutors announced the arrest of seven suspects in the massacre of 72 migrants in August.
President Felipe Calderon's office issued a statement saying he "energetically condemned" the slaying of the mayor of El Naranjo - the third mayor to be killed in Mexico in less than a month.
Amid the violence, U.S. Secretary of State Hillary Rodham Clinton said that Mexico is "looking more and more like Colombia looked 20 years ago, where the narco-traffickers control certain parts of the country, not significant parts."
Her comments raised hackles in Mexico.
"Of course we do not agree with the statement in this regard, given that there are very important differences between what Colombia faced then and what Mexico faces today," Mexican government security spokesman Alejandro Poire said.
Mexican officials say drug cartels are not allied with domestic rebel insurgencies, do not have political influence or following and do not exercise formal control of large swaths of the country, as they did in Colombia in the 1980s and 1990s.
But attacks like Wednesday's shooting death of El Naranjo Mayor Alexander Lopez Garcia suggest cartels are targeting civilian government in Mexico.
The San Luis Potosi state prosecutors' office said Lopez Garcia was killed by a squad of four hitmen who pulled up in a vehicle.
Two of the attackers burst into Lopez Garcia's office and killed him before fleeing. The rural township of about 20,000 people borders the violent-wracked state of Tamaulipas, where 72 migrants were massacred by drug gunmen in August.
There was no immediate information on the motive in the attack, but the style of the slaying resembles methods used by Mexico's drug cartels.
Lopez Garcia wa the third Mexican mayor slain in the last month. On Aug. 29, the mayor of a town just across the state line in Tamaulipas was shot to death and his daughter wounded. The mayor of Santiago, a town in the neighboring state of Nuevo Leon, was found murdered Aug. 18, a crime for local police officers allied with a drug gang are suspected.
Clinton made her statements Wednesday in Washington at the Council on Foreign Relations, where she said drug cartels are "morphing into, or making common cause with, what we would consider an insurgency in Mexico and in Central America."
"These drug cartels are now showing more and more indices of insurgency," Clinton said. "All of a sudden, car bombs show up which weren't there before."
Clinton also suggested that "we need to figure out what are the equivalents" for Mexico and Central America of the U.S. Plan Colombia - in which American special forces teams train Colombian troops and U.S. advisers are attached to Colombian military units.
Mexico has long rejected allowing U.S. troops on its soil, except for a single symbolic presence: Mexico's Senate has authorized a U.S. detachment to march in next week's Bicentennial parade.
Clinton acknowledged Plan Colombia was controversial, but added that "there were problems and there were mistakes, but it worked."
Later, U.S. Assistant Secretary of State Arturo Valenzuela said Clinton's comments shouldn't be misinterpreted.
"What we are concerned about is the fact that you see the development of phenomena of car bombs that can affect innocent people and these are terrorist acts, you can define them as terrorist acts," Valenzuela said.
"But the term insurgency should not be viewed in the same way we would refer to a Colombian insurgency. Not an insurgency of a militarized group within a society that is attempting to take over the state for political reasons."
He said what was happening in Mexico is an escalation of violence by criminal organizations, not an insurgency.
Also Wednesday, the Mexican government announced that marines had arrested seven gunmen suspected of killing 72 Central and South American migrants last month in the worst drug cartel massacre to date.
Four of the suspects were arrested after a Sept. 3 gunbattle with marines, and the other three were captured days later, spokesman Alejandro Poire said at a news conference.
Poire alleged the seven belong to the Zetas drug gang, but he gave no further details on their identities or what led to their arrests.
Investigators believe the migrants were kidnapped by the Zetas and killed after refusing to work for the cartel.
The arrests "will help determine exactly what happened in San Fernando, Tamaulipas, and it's a significant step toward ending the impunity surrounding assaults on migrants by organized crime," Poire said.
A total of eight suspects are now in custody: Marines arrested a teenager after a shootout with gunmen at the ranch the day they discovered the bodies. Three gunmen were killed during that battle.
In addition, marines last week found the bodies of three other men suspected of participating in the massacre after an anonymous caller told authorities where to find them. Officials say they have no information on who made the call, but in the past drug gangs have handed over suspects in especially brutal killings that draw too much attention.
A Honduran man who also survived the slaughter and is under police protection in Mexico later identified the three dead men as having been among the killers.
The latest arrests were announced one day after authorities found the bodies of two men believed to be those of a state detective and a local police chief who participated in the initial investigation of the massacre.
The two officers went missing a day after the migrants were found in San Fernando, a town about 100 miles south of Brownsville, Texas.
The Tamaulipas state Attorney General's Office said identification documents found on the bodies matched those of the missing officials, state detective Roberto Suarez Vazquez and Juan Carlos Suarez Sanchez, who was head of the Public Safety department of San Fernando.
The two bodies were found in a field about 30 miles northeast of San Fernando.