MEXICO CITY, Feb. 20 -- Federal law enforcement officials have arrested an assistant state prosecutor suspected of involvement in the murder of a state political party leader, a federal drug investigator and two pilots in the violence-torn border state of Durango.

The prosecutor, Hugo Armando Reséndiz Martínez, is also being investigated for allegedly protecting drug cartels and funneling advance notice of raids and investigations to notorious traffickers Sergio "El Grande" Villareal and Arturo "El Chaki" Gonzalez Hernández, the Mexican attorney general's office said in a statement late Monday.

Mexican leaders have long been accused in the United States of failing to crack down on corrupt public officials in the northern border states, where drug cartels are suspected of using bribes and coercion to control police officers, judges and city leaders. Victor Clark, an organized crime analyst and human rights activist in Tijuana, said Tuesday that it remained to be seen whether Reséndiz Martínez's arrest signaled a new approach to drug corruption.

"It could be an isolated event," Clark said. "Or it could be a way of sending a message to the rest of the prosecutors that the federal government could come after them."

The investigation of Reséndiz Martínez, 63, led to a mass grave where authorities found the body of Jaime Meraz Martínez, director of the Durango branch of the Democratic Revolutionary Party, or PRD, as well as the bodies of the federal drug investigator and the two pilots, the attorney general's office said. Investigators said one of Reséndiz Martínez's weapons might have been used to kill Meraz Martínez.

The prosecutor was immediately fired, but has not been charged.

Drug cartels have been suspected in more than 2,000 killings in the past year in Mexico. President Felipe Calderón, who took office in December, has sent federal troops to several states plagued by drug killings. But the violence has continued with almost daily news of gruesome killings, both in the border states and in resort areas, such as Acapulco.

Even as federal prosecutors were announcing the arrest of Reséndiz Martínez, a Mexican congressman was being treated for bullet wounds suffered in an assassination attempt in the border city of Nuevo Laredo, scene of some of the most intense battles between rival drug gangs. Congressman Horacio Garzaof the Institutional Revolutionary Party was in stable condition after being shot Monday in an attack that cost the life of his driver, Hector Morales Juarez.

Such assaults are common in Nuevo Laredo and other border cities. Calderón has sent troops to Tijuana and Monterrey.

Mexican cartels have grown exponentially in the past two decades and now might rival the wealth and power of the Colombian cartels of the 1980s. Clark said that growth has been fueled by the development of "solid" relationships between drug traffickers and local officials, making the task of combating the cartels all the more difficult.

"These are not transitory or fragile relationships," Clark said. "These groups have a lot of money to buy protection."