A car bomb exploded outside the studios of a top TV broadcaster early on Friday, days after marines found the bodies of 72 people gunned down in Mexico's escalating war with powerful drug cartels.

The blast, the second bomb planted in a vehicle this month in the city of Ciudad Victoria near the Gulf of Mexico and the third in Mexico since late July, caused no casualties but damaged buildings.

The explosion came the same day as authorities discovered the body of a police officer who was investigating the immigrant massacre, which has been linked to Mexico's drug war.

Friday's explosion, apparently part of a growing campaign of intimidation against the media in Mexico, left little more than the car's engine and front chassis near the studio of broadcaster Televisa in the capital of Tamaulipas state, located 220 miles (350 kms) south of the Texas border.

Televisa did not give details of the blast and it was unclear what explosives were used or how the bomb was detonated. No group was immediately blamed for the explosion.

In July, suspected drug cartels planted explosives in a vehicle, the first attack of its kind since Calderon took office, in the violent city of Ciudad Juarez. Four people were killed.

Another vehicle-borne bomb earlier this month in Ciudad Victoria did not cause any deaths.

More than 28,000 people in Mexico have died in drug violence since President Felipe Calderon launched his war on drugs in late 2006. As gruesome attacks become more common, Calderon is seeking to convince civic leaders, businessmen and opposition politicians that his war is making headway.

MEDIA IN CROSS-HAIRS

The explosion was at least the fourth apparent attack on Televisa studios in northern Mexico since last year, when drug hitmen threw a grenade at its studios in Mexico's business capital Monterrey. Grenades also were thrown at Televisa in Monterrey and Matamoros across from Brownsville, Texas, earlier this month, police said. No one was hurt in the attacks.

They want to sow terror in society and they are going for symbols. Televisa has a big presence at a local level in this region, said Alberto Islas, an independent security analyst with crisis management firm Risk Evaluation. The attack is not against the company but against freedom of expression.

At least 30 journalists have been killed in Mexico since 2006, according to Mexican media. The country is among the world's most dangerous for reporters, the U.S.-based Committee to Protect Journalists says.

INVESTIGATOR KILLED

Tamaulipas state has become one of Mexico's bloodiest drug flashpoints since the start of the year as rival hitmen from the Gulf cartel and its former armed wing, the Zetas, fight over smuggling routes into the United States.

Gunmen murdered a popular candidate for state governor in June in Mexico's worst political killing in 16 years.

The car bomb comes after the bodies of 72 people were found on Tuesday at a remote ranch about 100 miles (160 km) from Ciudad Victoria. It was the worst massacre since Calderon sent 45,000 troops and thousands of federal police to fight drug gangs in late 2006.

The victims, believed to be trying to make their way to the United States from Central and South America, appeared to have been blindfolded and bound before being gunned down. A military source in Tamaulipas said the Zetas were behind the killings.

Today these acts have shaken both the national and international conscience, Calderon said at a public security forum in Mexico City. We will continue to support states and municipalities with federal forces to counter the action of criminals.

Two senior police officers who were investigating the massacre were abducted Thursday and the body of one of them was dumped on a rural road on Friday, El Universal daily said on its website.

(Reporting by Reuters newsroom in Ciudad Victoria and additional reporting by Cyntia Barrera in Mexico City; Writing by Robin Emmott; Editing by Bill Trott)

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