MEXICO CITY -- In a symbol of escalating violence by drug traffickers in Mexico, gunmen guarding an opium-poppy plantation shot down two police helicopters, killing all five agents aboard.

The shooting Monday suggested that Colombian-style drug violence may be spreading and raised concerns that the government might begin militarizing the mountains of southern Mexico.

Estuardo Bermudez, Mexico's top anti-drug prosecutor, said drug-eradication flights have been fired on from the ground 29 times in the last two years.

But while helicopters have been riddled with bullets in the past, none has ever been brought down by ground fire and no pilots or crew have been killed before this week.

"We pay tribute to the work of these public servants, who every day face the unknown and risk their lives in remote areas," Bermudez told a news conference.

Colombia, the only other country in the hemisphere where anti-drug flights regularly come under fire, had two choppers brought down over the last two years.

The attack near Tlapa, 130 miles southeast of Mexico City, was yet another example of the battle-style tactics used by drug growers to protect their opium plantations.

At least five times in the last year, they hung steel cables across drug fields to try to snare the rotors of crop-spraying helicopters, Bermudez said.

While most heroin sold in the United States still comes from Colombia, opium poppy cultivation has grown rapidly in Mexico. U.S. officials have said Mexico's clandestine poppy fields are among the best-hidden in the world, often in mountaintop jungle clearings.

Bermudez said there was no evidence the gunmen involved in Monday's attack belonged to any organized guerrilla or paramilitary groups -- the main actors in Colombia's violence.

But the prospect of a possible Colombian-style conflict worried many analysts.

"This kind of attack could serve as some sort of justification for alleging a link between guerrilla groups and drug traffickers," said Luis Astorga a drug-policy expert at the National Autonomous University of Mexico.

Colombian left-wing guerrillas and right-wing paramilitary forces both profit from, and protect, cocaine traffickers there.