|More than 34,000 people have been killed so far in Mexico's drug-cartel violence [AFP]
Mexican authorities have found an unmarked grave in the northern state of Durango containing 13 bodies, three of them beheaded, officials say.
The bodies - found in a residential area of the state capital, also named Durango - were in an advanced state of decomposition, state officials said in a statement on Saturday.
The corpses have not yet been identified, and were taken to a morgue for autopsies, the Durango state prosecutor's office said in a statement.
The grave find came after authorities in the northwestern border state of Tamaulipas unearthed mass graves holding more than 145 corpses in recent days.
Durango is part of the so-called Golden Triangle of northern states - along with Sinaloa and Chihuahua - that has seen a sharp rise in unrest since authorities began a crackdown on drug traffickers who use the region to grow marijuana and opium poppies, and transport their wares.
The Tamaulipas killings - apparently of passengers on public buses in the region - are being blamed on the drug gang, the Zetas.
Durango has been a battleground in a bloody turf war between the Sinaloa cartel and the Zetas gang.
Authorities on Thursday arrested 16 police officers suspected of protecting four members of the Zetas cartel, which has been blamed for the massacres.
Sinaloa gang arrests
Separately, police arrested 13 alleged members of the Sinaloa cartel drug gang in the city of Guadalajara on Friday.
Police said they arrested the men, who were travelling on board a 4x4 vehicle, after receiving an anonymous tip.
Carlos Najera, the public security minister, said that the men had been previously arrested and released.
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"They are hitmen. They could've been taking part in several homicides there have been in the metropolitan area and kidnappings, but that will have to be established by the prosecutor's office. The alarming thing is that several of them had already been arrested in possession of weapons and drugs," Najera said.
"At least three of them were arrested by police in March of this year in Zapopan and they are now on the street again with weapons and drugs. So we see that the policemen's effort is, disgracefully, lost."
Carlos Quiroz, one of the suspects, denied participating in any criminal activity or being a part of the Sinaloa Cartel.
In another development, Mexico's army announced that it had captured a man who has allegedly confessed to involvement in the killing of seven people, including a well-known poet's son.
The killing of Juan Francisco Sicilia on March 28 sparked demonstrations across the country against the violent drug war in the country, which has claimed more than 34,000 lives so far.
Circumstances surrounding the arrest, however, cast doubt over whether Rodrigo Elizalde Mora, the suspect, was coerced into the confession.
The army says that Mora was captured in the central city of Cuernavaca, where the killings occurred, on Thursday.
He confessed to working for the South Pacific Cartel, led by Hector Beltran Leyva, and to help kill Sicilia and a group of friends, an army statement said.
Mora, however, appeared before reporters badly beaten, and said he had been kidnapped by four unidentified men in a 4x4 vehicle, who had then turned him over to the authorities.
"They picked me up and they beat me," he told local media.
The chain of events is similar to a case in 2008, when three suspects were quickly arrested for a grenade attack on a crowd celebrating Mexico's independence.
Eight people died in that attack, and the suspects who were arrested said they were kidnapped by a drug cartel, beaten and left for the authorities.