|A forensic team investigates crime scene where two decapitated bodies were found in a car in Mexico City [Reuters]
Two US citizens are reported to be among four people killed in an attack by armed men on a vehicle in the Mexican city of Ciudad Juarez.
In a separate incident also blamed on drug violence, two severed human heads have been found in the capital, Mexico City.
Although the dual findings mark another violent weekend in Mexico's escalating drug war, some schools in the Pacific coast resort of Acapulco reopened after months of closure due to violence.
The El Paso Times reported on Tuesday that the US consulate in Ciudad Juarez had identified the US citizens killed over the weekend as 19-year-old Pablo Noe Williams and his mother, 35-year-old Rosa Williams.
The others killed were identified as 24-year-old Alberto Nieto Nieto and 21-year-old Alma Yesenia Flores.
The Chihuahua state attorney general's office said the 4WD vehicle with Texas plates that the four were in on Friday evening in Juarez was hit with bullets from assault rifles.
Police in Mexico City found the severed heads on a street near a major military base, a tactic of feuding drug gangs that has long affected other parts of the country while largely sparing Mexico City.
Decapitations are frequently carried out by gangs in violence-plagued cities such 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 Miguel Mancera, Mexico City's attorney general, said in a statement that one of the heads found on Monday had been placed on the bonnet of a 4WD 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 on Monday that the heads were accompanied by a note referring to the "Mano con Ojos'' or "Hand with Eyes'' drug gang.
The organisation 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 Hand with Eyes gang formed after the arrest of Edgar Valdez, aka "La Barbie'', an 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 on Monday, about 120 soldiers patrolled streets around schools in the city's rougher neighbourhoods, 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, 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 neighbourhoods far from the city's glittering resorts, few schools were open on Monday.