With helicopters overhead, heavily armed patrols in armoured personnel carriers, trucks and jeeps swept though towns and cities in the border region a day after the bodies of 72 people were found in an empty building at a remote ranch.
The victims, Central and South American migrant workers, appeared to have been blindfolded and bound before they were lined up against a wall and gunned down.
Photographs showed bloodstained bodies heaped on the ground at the ranch in Tamaulipas state, which has become the scene of some of Mexico's worst drug violence as the Gulf cartel and a spinoff group, the Zetas, fight over smuggling routes.
Officials said investigators were still examining the scene, about 90 miles (150 km) from the Texas border, as many of the victims had no identification documents. They had not yet removed all the bodies, which may be flown to state capital Ciudad Victoria.
The United States condemned the heinous crimes and offered its support in the drug war. They are dangerous, they are attempting to undermine the democratic institutions of Mexico, and that's why we pledge to ... defeat these cartels, State Department spokesman P.J. Crowley said in Washington.
The sole survivor of the massacre, an Ecuadorean man, escaped the ranch on Monday after being shot, and told authorities about the killings. He said his fellow victims included Brazilians, Hondurans, Salvadoreans and Guatemalans.
Diplomats from Brazil, Ecuador, El Salvador and Honduras flew to the crime scene on Thursday. Central and South American nations condemned the massacre and Ecuador said it was urging Mexico to protect the survivor, Luis Freddy Lala Pomavilla.
Migrants trying to slip into the United States from Mexico are increasingly at risk of kidnapping and extortion by drug gangs that operate with impunity in parts of the country's northern reaches, police and analysts say.
A lot of these migrants are mustered in or forced in under extortion to be part of the drug mule process. They'll be threatened to carry drugs to the United States, said drug trade analyst Fred Burton at security consultancy Stratfor.
Security forces were fired on when they approached the ranch on Tuesday, and in the ensuing firefight marines killed three gunmen and arrested another. Several suspects escaped.
More than 28,000 people have been killed in drug violence since President Felipe Calderon launched his war on the cartels when he took office in 2006. Calderon has vowed to push ahead with the crackdown but has warned of more violence ahead.
A military source in Tamaulipas said the Zetas, a brutal gang led by Heriberto Lazcano, a former Mexican soldier known as the Executioner, were behind the killings.
Ruling party politicians defended the government's crime record on Thursday, saying the massacre signalled desperation within criminal groups under pressure from the government.
But activists denounced the government for failing to protect migrants, saying thousands of undocumented travellers suffer abuse at the hands of criminal gangs every year.
Interior Minister Francisco Blake pledged to bring the killers to justice and protect migrant workers.
While most of the drug war bloodshed has been confined to gang members and security forces, violence is spreading to parts of the country once deemed peaceful, including Mexico's business capital Monterrey, worrying investors and Washington and scaring off some tourists. More than 150 federal police flew to Monterrey on Thursday, taking the deployment of elite officers to more than 300 this week to tackle drug crime.
A grenade explosion at a bar in the famed beach resort of Puerto Vallarta on Wednesday night wounded 16 people, four seriously, the Jalisco state prosecutor's office said.
Puerto Vallarta is a popular Pacific coast destination for sun-seeking foreign tourists but the bar is located away from the tourist area. No foreigners were wounded in the blast.
Violence in Jalisco, where Puerto Vallarta is located, has increased since security forces killed top trafficker Ignacio Coronel, who controlled the drug trade in the state, in July.
(Additional reporting by Alexandra Valencia in Quito; Miguel Angel Gutierrez in Mexico City and Robin Emmott in Monterrey; writing by Robert Campbell and Robin Emmott; editing by Missy Ryan and Mohammad Zargham)