An entire 20-man police force resigned in a northern Mexican town after a series of attacks that killed the police chief and five officers over the last three months, state officials said Thursday.

The officers' resignation Thursday left the 13,000 people of Ascension without local police services, Chihuahua state chief prosecutor Carlos Manuel Salas said. State and federal police have moved in to take over police work, he said.

The mass resignation appeared to be connected to a Tuesday attack by gunmen that killed three of the town's officers, Salas said.

But it wasn't the first deadly attack on the police department this year.

In mid-May, police chief Manuel Martinez, who had been in office just seven months, was gunned down with two other officers on a nearby highway. The three had been kidnapped a day before police found their bodies riddled with bullets in the back seat of a sedan.

The town's police force was relatively new.

Angry residents had led authorities to replace the entire force last September after the mob killings of two teenagers who had allegedly kidnapped a girl from a seafood restaurant. People claimed police officers were aiding drug gangs.

Martinez, with his new police force, had said he wanted to end the kidnappings and extortions that have terrorized the town where people grow green chili and cotton.

The new police in Ascension had installed a telescopic camera in the town's plaza that rotated, giving officers at the station the ability to zoom on a site as far as the outskirts of town.

In addition, townspeople helped police dig a broad ditch around the town to prevent criminals from escaping on back roads.

Ascension is southwest of Ciudad Juarez, the border city across from El Paso, Texas, that is one of Mexico's most violent cities. The state of Chihuahua has had the most homicides blamed on organized crime and drug trafficking since the government's anti-drug offensive began in December 2006.

Elsewhere, the Defense Department announced that a 19-day offensive in northern states against the Zetas drug cartel had resulted in the shooting deaths of 30 alleged criminals and a soldier.

The army said that among those killed was Jorge Luis de la Pena, the Zetas boss for Nuevo Laredo, the city across the Rio Grande from Laredo, Texas.

Troops also detained 196 people in different cities during operation "North Lynx."

The Zetas gang, known for its viciousness, has been fighting its former ally, the Gulf cartel, in Mexico's north since early 2010.

Near the northern industrial hub of Monterrey, police found the bodies of two men each hanging by an ankle from a pedestrian bridge. Officers said a witness reported that gunmen strung up the men alive and then shot them.

Such grisly displays at bridges have become common in and around Monterrey as well as in other Mexican cities torn by drug violence.

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Associated Press writer Adriana Gomez Licon in Mexico City contributed to this report.