MEXICO CITY — The third Mexican mayor in a month was slain by suspected drug gang hitmen on the same day the U.S. secretary of state raised hackles in Mexico by saying the country is "looking more and more like Colombia looked 20 years ago."

Hillary Rodham Clinton and other U.S. officials pointed to Mexican drug cartels' use of three car bombs, a tool once favored by cartel-allied rebels in Colombia, as evidence that the gangs "are now showing more and more indices of insurgency."

While the Mexican government quickly condemned the killing of the mayor of the northern town of El Naranjo, it rejected the comparison with Colombia, where the Medellin drug cartel waged a full frontal assault on the state, endangering its very integrity with attacks on police, politicians and judges and terror attacks against civilians.

More worrisome to Mexican legislators, Clinton suggested the United States was looking to implement some type of Plan Colombia for Mexico and Central America, referring to a U.S. anti-drug program in which American special forces teams trained Colombian troops and U.S. advisers are attached to Colombian military units.

The reaction was swift.

Mexico — which has suffered at least three U.S. invasions — has always rejected allowing American 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.

"Starting right now, we have to say this clearly. We are not going to permit any version of a Plan Colombia," said Sen. Santiago Creel, a member of President Felipe Calderon's National Action Party. "We cannot permit a Plan Colombia in Mexico."

Sen. Ricardo Monreal of the leftist Labor Party said U.S. aid to Colombia hadn't stopped drug trafficking there. "Whoever thinks Colombia is a cure-all, and if the United States thinks it is necessary to apply the same model to us they applied to Colombia, they are mistaken," he said.

Plan Colombia has been widely credited for helping Colombia diminish the rebel threat, but critics say it has not put a significant dent in the drug trade.

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."

Clinton also suggested that "we need to figure out what are the equivalents" for Mexico and Central America of Plan Colombia, acknowledging "there were problems and there were mistakes, but it worked."

Mexican cartels are becoming increasingly violent — federal police reported Wednesday they had found four bodies in a clandestine grave linked to arrested U.S.-born drug hitman Edgar Valdez Villarreal, alias "La Barbie" — and are carrying out more attacks on government officials in Mexico.

Hooded gunmen burst into Mayor Alexander Lopez Garcia's office in the northern Mexico state of San Luis Potosi on Wednesday and shot him to death.

President Felipe Calderon's office issued a statement condemning the killing — the third mayor slain in less than a month — calling it a "cowardly and criminal" act.

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.

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.

The San Luis Potosi state prosecutors' office said Lopez Garcia was killed by a squad of four hitmen. 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.

On 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.

An eighth suspect already was 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.

A Twitter account linked to Calderon's website said two youths aged 14 and 17 had also been detained for allegedly participating in the massacre, but offered no details. The president's office was not immediately available to clarify the report.

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.

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Associated Press Writer Mark Stevenson contributed to this report.