Two offices of the El Norte newspaper were attacked with grenades in Monterrey, the capital of the northern Mexican state of Nuevo Leon, damaging the buildings but not causing any injuries, the State Investigations Agency, or AEI, said.

The first attack occurred early Tuesday and targeted the newspaper's office on Avenida Revolucion in the southern section of the city.

A subject entered the parking lot and threw a grenade, leaving the pin on the ground, the AEI said.

No one was working in the offices because it was early, but some employees were in the rear of the building.

The second attack happened just before 4:00 p.m. at El Norte's office in Libertad, a neighborhood on the east side of Monterrey.

Gunmen threw a grenade at the office and fired at least 10 shots at the building with an AK-47 assault rifle.

Employees were working in the building, but no one was hurt.

Two women, however, had to be treated for shock, officials said.

Municipal police, AEI agents and army troops went to the scene, as well as investigators trained to handle explosives cases.

This was the fourth attack involving grenades on El Norte's facilities, the newspaper said.

El Norte said it was also attacked on Sept. 20, 2010, Jan. 10, 2011, and March 31, 2011, but no one was injured in any of the attacks.

The Los Zetas cartel has been battling the Gulf cartel for control of the Monterrey metropolitan area and smuggling routes into the United States.

A total of 324 murders were registered in Nuevo Leon in the first two months of the year, a record for a two-month period in the state.

The wave of drug-related violence in Nuevo Leon claimed the lives of 2,003 people in 2011, official figures show.

Heriberto Lazcano Lazcano, known as "El Lazca," deserted from the Mexican army in 1999 and formed Los Zetas with three other soldiers, all members of an elite special operations unit, becoming the armed wing of the Gulf drug cartel.

After several years on the payroll of the Gulf cartel, Los Zetas, considered Mexico's most violent criminal organization, went into the drug business on their own account and now control several lucrative territories.

Mexico's drug war death toll stood at 47,515 from December 2006 to Sept. 30.

The murder total has grown every year of President Felipe Calderon's military offensive against the well-funded, heavily armed drug cartels.

Unofficial tallies published in December by independent daily La Jornada put the death toll from Mexico's drug war at more than 50,000. EFE