Mexico City  - Police in the northern Mexican town of Ascension, near the US border, received a parcel containing a cooler packed with four human heads, Mexican media reported Tuesday.

The parcel arrived last week, but it was not opened until Monday because it was initially thought to have been sent by mistake to the police command in Ascension, some 150 kilometres southwest of Ciudad Juarez.

The cooler was labelled "Vaccines, delicate material, handle with care," so police first contacted local hospitals and health centres to check whether they had ordered vaccines.

Since no one claimed the package, they proceeded to open it. Police reportedly found the heads of four unidentified men, aged 25- 35, inside the package.

Last week 10 men were kidnapped by armed men in the area, but no ransom demands were made. The authorities were investigating whether the two cases might be related.

Some 3,500 people have been killed so far this year in incidents linked to organized crime in Mexico.

In the past two years, the government has deployed thousands of Army and federal police officers in an effort to combat drug gangs. (dpa)