Homicides related to organized crime jumped 47 percent in 2008, Mexico's attorney general said Friday in a rare confirmation of how bad violence has become. Police later made two gruesome discoveries in northern Mexico. Five bodies - two of them decapitated - were found wrapped in blankets in a city on the border with Texas, along with two heads in sacks. In another state, police found four severed heads in ice chests along a highway. Attorney General Eduardo Medina Mora told Radio Formula that 1,378 people have been killed so far this year, compared with 940 in the same period last year.

The statistic reflected what many in Mexico already knew: Drug-related killings have soared in recent months.

But the details were the first official snapshot on the rise in killings. The Mexican government has been reluctant to release homicide statistics, leaving the public to rely on informal tallies by the news media.

Medina Mora broke that silence, saying 4,152 people have been killed since President Felipe Calderon took office in December 2006 and declared war on drug cartels that controlled entire regions of Mexico. About 450 of those were police, soldiers, prosecutors or investigators.

Medina Mora said many of the recent killings have been concentrated along the U.S. border, while homicides in the central part of the nation have subsided.

The government says the violence reflects drug gangs' desperation amid the nationwide crackdown, carried out by more than 20,000 soldiers and federal police.

"Evidently when they are cornered and weakened, they have to respond with violence," Medina Mora said.

Analysts say recent arrests have created a power vacuum and gangs are battling for valuable drug routes and territory.

One of the hardest-hit cities is Ciudad Juarez, where the five wrapped bodies were found on a street corner. A message found with the corpses read "this is what happens to traitors who align themselves with Chapo Guzman," a reference to reputed Sinaloa drug cartel chief Joaquin Guzman.

A recent e-mail warning of a weekend bloodbath has alarmed many residents of the city across from El Paso, Texas.

Nobody seems to know who wrote the e-mail, which says gunmen will fire at malls, restaurants and other public places in "killings all over the city." But many people have forwarded it to friends.

Juarez Police Chief Roberto Orduna said the threats must be taken seriously and issued a news release Thursday assuring residents that police would be more vigilant.

More than 200 people have been killed so far this year in Juarez.

Police found the four severed heads in ice chests outside a highway convenience store in the northern state of Durango, along with unspecified threatening messages, the state government said. The bodies were not found, and the victims were not identified.

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Associated Press writer Marina Montemayor contributed to this report from Ciudad Juarez, Mexico.