MEXICO CITY (Reuters) - Mexican police on Sunday found nine decapitated bodies and the army identified eight soldiers who had died fighting powerful drug gangs and whose murders were seen as a brazen challenge to the government.

The bodies showed signs of torture. They were left on the side of a highway about an hour north of the tourist resort of Acapulco in the southern state of Guerrero, state police said.

Their heads were stuffed in a plastic bag and left outside a shopping center.

Mexico’s President Feline Calderon has deployed tens of thousands of troops and police since 2006 to take on drug cartels. The defense ministry vowed not to back down despite its latest losses.

“They are trying to scare the military. Regardless, the ministry promises to continue fighting,” it said in a statement.

The ministry released the names of eight decapitated soldiers but said one of them was recovered on December 9.

Drug killings throughout Mexico have more than doubled to over 5,300 this year, scaring off investment and tourists. The United States has sent hundreds of millions of dollars in aid to help its southern neighbor fight the cartels.

The Mexican army has made some prominent captures, but the cartels seem able to quickly replace their losses. Meanwhile, a growing number of police have been gruesomely murdered.

A note left with the severed heads warned of more decapitations, the state police said.