Plainclothes policemen and explosive experts work at the site of a car bomb attack in the municipality of Zuazua, some 40 km (25 miles) from Monterrey December 17, 2010. [Photo/Agencies]

MONTERREY, Mexico -- More than 140 inmates escaped via the main entrance of a prison near the US border on Friday in the biggest Mexican jailbreak since the government began its war on drugs four years ago.

Hours later, suspected hitmen blew up a car outside a police station near the business hub of Monterrey in the latest act of brinkmanship between drug gangs and officials.

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In a brazen move underscoring Mexico's weak prison system, inmates slowly filed out of the main vehicle entrance of a prison in Nuevo Laredo across from Texas early on Friday, two police sources in northern Tamaulipas state said.

Later on Friday in the small town of Zuazua on the northern outskirts of Monterrey, an SUV exploded, injuring two people and knocking out power. It was the first such explosion near Mexico's richest city, a business center with close US ties.

While authorities declined to say if the two incidents were linked, Zuazua lies on the highway between Monterrey and Nuevo Laredo and the area has become a major flashpoint in the drug war since early this year, when a cartel split into rival factions.

The blast shattered windows and destroyed a car parked nearby but its impact was limited. The crumpled remains of the bomb vehicle were just visible from behind a police cordon.

Jorge Domene, a spokesman for Nuevo Leon state that includes Zuazua and Monterrey, blamed organized crime and said the explosion was aimed at intimidating police. "It is obvious this is a message to the authorities," he told Milenio TV.

for Nuevo Leon and the neighboring state of Tamaulipas in a wider cartel war across Mexico over smuggling routes into the United States and local criminal rackets. The war has killed more than 30,000 people since late 2006, according to official figures. Media reports put the sum at more than 33,000.

In a new tactic, suspected drug hitmen began using car bombs this year, first in Ciudad Juarez, Mexico's deadliest city across from El Paso, Texas, in July and then in Ciudad Victoria in Tamaulipas in August.

Mounting insecurity in Mexico is a threat to Latin America's No. 2 economy as investors question the safety of doing business. The violence is also a worry for Washington. US Secretary of State Hillary Clinton warned in September that Mexico was starting to resemble Colombia at the height of its cocaine-fueled insurgency in the 1980s and 1990s.