The El Manana newspaper in the northern Mexico border city of Nuevo Laredo says it will stop covering violent criminal disputes after suffering a second grenade attack against its offices in two months.

Other northern Mexican newspapers have quietly adopted similar policies of not covering drug cartel violence to protect their staffs against threats and violent attacks including kidnappings and murders carried out by gangs that either don't want their activities to appear in print, or are angered by coverage of their rivals.

A man reads newspapers in front of a bank in downtown Mexico City. Many Mexican newspapers have stopped writing about drug cartels to protect employees. (Tomas Bravo/Associated Press)

But El Manana's announcement Tuesday was unusual because it was public. The paper did not say who it thought was behind the attack earlier in the day, nor give a possible motive.

"We ask for the public's comprehension and will refrain, for as long as needed, from publishing any information related to the violent disputes our city and other regions of the country are suffering," the paper said in an editorial.

Nuevo Laredo, like much of Tamaulipas state, has been the scene of bloody battles between the Zetas drug gang and the Gulf cartel, supported by allies in the Sinaloa cartel. Nuevo Laredo is located across the border from Laredo, Texas.

"The company's editorial and administrative board has been forced to make this regrettable decision by circumstances we are all familiar with, and by the lack of adequate conditions for freely exercising professional journalism," according to the El Manana statement.

"We will only address the [violent crime] issue through the opinions of professional analysts who study the phenomenon in an intelligent and responsible way."

Also Tuesday, gunmen threw grenades and opened fire on two buildings belonging to the El Norte newspaper in the northern state of Nuevo Leon. No injuries were reported in those attacks, and that newspaper has not announced any change in coverage.

The newspaper reported Monday that employees of a local motor vehicle bureau were suspected of improperly providing thousands of licence plates that were used to make stolen cars appear to be legitimately licensed vehicles that were then sold to unsuspecting buyers.

El Manana and one of El Norte's offices, home to a weekly local supplement of the paper, both have been attacked before.

"It is appalling that three Mexican media outlets, two of which have been attacked repeatedly in the past, were violently targeted in a single day," said Carlos LaurDia, CPJ's senior program co-ordinator for the Americas. "Mexican authorities must fully investigate these crimes, provide protection to the outlets, and ensure that the journalists can work without fearing for their lives."

Since 2000, 81 journalists have been killed and 16 kidnapped in the drug war, the Mexican government's human rights commission says. The New York-based Committee to Protect Journalists says 48 journalists have been murdered or disappeared since President Felipe Calderon launched an offensive against drug cartels in December 2006.

Some of the Mexican papers that have made similar decisions not to cover drug violence continue to print crime sections, but fill them with stories about traffic accidents. Others have decided to protect themselves by covering homicides only by printing police statements, without investigating. Still others continue to file stories, but try to avoid mentioning the name of any specific gang or cartel.