MEXICO CITY — Gagged and bound, the bodies were dumped on a grassy roadside littered with trash. Most had been shot in the head, probably on the spot, judging from the spent shell casings. Some were carted there, already dead, authorities believe.

In what appears to be the largest single mass killing since Mexico's drug war exploded nearly two years ago, the bodies of 24 men were discovered about 30 miles outside this capital late Friday. The execution-style slayings were likely the latest battle between rival drug gangs, officials said Saturday.

The bodies were found in La Marquesa park, near a rest stop frequented by weekend travelers from Mexico City.

The drug war "knows no borders," said Enrique Peña Nieto, governor of Mexico State.

He said the slayings were part of the "insecurity that prevails, in generalized form, in the nation."

All of the dead men sported military-type haircuts and were dressed in T-shirts and shorts, apparel appropriate for weather warmer than that of Mexico City. Authorities suspect they came from the neighboring states of Michoacán or Guerrero, where fighting among drug gangs is especially relentless.

None of the men were believed to be active-duty soldiers, said Attorney General Eduardo Medina Mora.

There was speculation that either the killed or killers belonged to the Zetas, a ruthless band of hit men, many of them ex-military, working for one of the most powerful narcotics-trafficking syndicates.

The Attorney General's Office said in a statement that the dead ranged in age from 20 to 35. All had been shot in the head. Their hands and feet were tied with gray tape, also used to bind their mouths, said Octavio Campos, a spokesman for the state prosecutor's office.

"The initial investigation indicates some of the people had been executed before arriving at the site while others were executed there," Campos said. "This may be a settling of scores between organized criminal bands."

The attorney general's statement noted that none of the victims had been decapitated.

According to a tally kept by Reforma newspaper, 3,148 people have been killed in drug-related violence in 2008, with Friday the single most deadly day of the year.

Elsewhere in Mexico, 17 other people were killed in shootings and attacks. Many victims turn up headless, including 12 decapitated bodies discovered Aug. 28 on the Yucatan Peninsula. Kidnappings are also at epidemic levels.

Warfare that was once confined to lawless swaths of Mexico's border and along smuggling routes has spread throughout Mexico following President Felipe Calderón's decision to unleash the army against traffickers in December 2006.

Calderón maintains the turmoil is proof his crackdown is putting pressure on the criminal organizations that control a multibillion-dollar trade in cocaine, marijuana and heroin and who must compete for routes and dominance of the market.

He has dispatched 40,000 troops and 5,000 federal police officers into the streets as part of the campaign.

Analysts warn, however, that the escalating grisliness employed by traffickers and their henchmen is aimed at frightening Mexican citizens and eroding public support for the government's efforts. Polls indicate Mexicans, once able to dismiss the bloodshed as something that played out among the criminals, are increasingly worried as the violence spreads to new areas and claims innocent victims.

Calderón, speaking at a ceremony Friday at Mexico's Military College, called on Mexicans to forge and sustain a united front against their criminal "enemies" to save the state from being overrun.

"In this war," he said, "there can be no truce because we will rescue, one by one, the towns, cities and public spaces in power of the criminals and return them to our children, citizens, mothers, grandparents. ... We will continue using all — all — resources within our reach to recover the state's control and its territory, on all fronts."