A group of armed gunmen believed to be part of a powerful drug cartel in Mexico ambushed a police convoy in the western state of Michoacan, killing at least 13 officers and wounding nine.

Some 42 state police officers had gone to the township of Aguililla on Monday to enforce a judicial order when they were met with more than 30 gunmen believed to be from the Jalisco New Generation, one of Mexico’s most powerful cartels at the moment.

Officials said the gunmen – some in vehicles that were apparently armored – opened fire on the police convoy with .50 caliber sniper rifles and AR-15 and AK-47 assault-style rifles.

At least 13 officers were killed, some of their bodies still inside the patrol trucks when the vehicles were set afire.

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The attack – the worst on Mexican law enforcement in years – comes at a time when violence attributed to drug cartels has spiked in recent months.

“No attack on the police will go unpunished, and this was a cowardly, devious attack because they laid an ambush in this area of the road,” Gov. Silvano Aureoles said.

Images published in Mexican media showed vehicles burning in the middle of a highway and messages apparently signed by Jalisco New Generation. A purported leader of the cartel was killed by Michoacan police less than a week ago.

Journalists also photographed two gutted patrol cars at the entrance of the El Aguaje – a town in Aguililla – surrounded by hundreds of bullet casings. Two police trucks were towed away.

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Michoacan, an important avocado-growing state, has recently has seen a jump in violence that has brought back memories of the bloodiest days of Mexico’s war on drug cartels between 2006 and 2012.

El Aguaje is considered to be a strategic battleground between Jalisco New Generation and a splinter group of the Knights Templar called Los Viagras, the BBC reported.

In August, police found 19 bodies in the town of Uruapan, including nine hung from a bridge. Later, an area roughly 45 miles north of Aguililla was the scene of fierce clashes between members of Jalisco New Generation and regional self-defense groups.

In 2013, civilian groups faced with what they said was state inaction armed themselves in Michoacan to fight the Knights Templar cartel, one of whose bases was in Aguililla. They said they took up arms to defend themselves from kidnappings, extortion, and killings by cartels. But some of the self-defense or vigilante groups later became infiltrated by cartels and gangs.

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After Monday’s attack, the streets were nearly empty and the area was reinforced by federal and state security forces, which set up checkpoints to hunt fo the assailants.

Besides avocado orchards, Michoacan for decades has been known for marijuana plantations and the making of methamphetamine. It is home to the port of Lázaro Cárdenas, a key entry point for precursor chemicals used to make synthetic drugs.

The Associated Press contributed to this report.