Police in Mexico City found two severed human heads on a street near a major military base Monday, a grisly tactic of warring drug gangs that has long affected other parts of the country while largely sparing the capital.

Decapitations are frequently carried out by gangs in drug-violence-plagued cities such as the Pacific coast resort of Acapulco and in northern Mexico, often to intimidate or threaten rivals.

But it was the first multiple decapitation in the capital since January 2008, when two heads were found near the city's international airport. Two heads were also found in the same vicinity in December 2007. Those killings were believed to be related to a drug shipment that had been seized at the airport.

The office of Mexico City Attorney General Miguel Mancera said in a statement that one of the heads found Monday had been placed on the hood of a sport utility vehicle, and the other was found on a nearby sidewalk.

The heads were found just before dawn on the side of a busy ring road across from the army's headquarters at Military Camp 1.

Mancera was quoted by local news media as saying Monday that the heads were accompanied by a note referring to the "Mano con Ojos" or "Hand with Eyes" drug gang. The organization has been active in the State of Mexico, which borders Mexico City, and in some southern districts of the capital. Mancera gave no details of what the message said.

The heads were found on a busy ring road across from the army's national headquarters at Military Camp 1.

The Hand with Eyes gang formed after the arrest of Edgar Valdez, aka "La Barbie," a top assassin for the Beltran Leyva cartel until he was detained in 2010.

The alleged leader of the Hand with Eyes, Oscar Osvaldo Garcia Montoya, 36, was arrested in July and told prosecutors he helped carry out or ordered more than 600 killings.

In Acapulco Monday, about 120 soldiers patrolled streets around schools in the city's rougher neighborhoods, but that still did not convince students, teachers and parents to reopen all of the 460 schools that had closed because of extortion demands and threats. That number represents about one-third of all schools in the city of about 800,000.

Many of the mainly primary schools had been closed since late August, when students were scheduled to return to classes after summer vacations.

Banners, handwritten signs and other threats had appeared around schools, in some cases demanding that teachers hand over part of their pay as protection money.

The state government reached an agreement last week to start reopening schools gradually Monday, in exchange for increased security patrols, the installation of alarm buttons and promises to investigate specific cases of threats or extortions.

But in working-class neighborhoods far from the city's glittering resorts, few schools reopened Monday.

Arturo Martinez, a spokesman for the government of Guerrero state, where Acapulco is located, said that 90 percent of schools reopened, but acknowledged that at many it was a 'soft' opening, with teachers taking stock of the classrooms, parents showing up for meetings and other preparations, but no actual classes.

Acapulco has been the scene of bloody turf battles between the Zetas, the Knights Templar and remnants of the Beltran Leyva cartel.

In the home state of the Zetas cartel leader Heriberto Lazcano, Hidalgo police chief Damian Canales said officers captured four alleged members of the gang believed to have participated in the killings of 70 people, most whose bodies were disposed of by being dissolved in vats.

Canales said on Monday that one of the suspects, 21-year-old Javier Rodriguez, told agents he shot 20 members of the Gulf Cartel, which is engaged in a fierce battle with the Zetas. Canales said Rodriguez served in the Mexican army.

Authorities did not say when the operation that netted the arrests took place.