Omar Pimentel, 38, gave his letter of resignation to Mayor Daniel Peña late Wednesday, hours after police found three charred bodies on the side of a road leading into Nuevo Laredo.

Police did not immediately know who was responsible for the killings.

"I thank the mayor for the opportunity to collaborate in the 2005-2007 administration through such an important and key post," Pimentel wrote.

Pimentel, who surrounded himself with at least a dozen bodyguards toting automatic rifles, did not say why he was resigning and could not immediately be reached at his office Thursday.

He was named police chief in July after serving as director of the city´s police academy.

TEMPORARY STAND-IN

On Thursday, Peña announced Nuevo Laredo police administrator Guillermo Landa will serve as interim police chief.

Pimentel´s predecessor, Alejandro Domínguez, was gunned down just seven hours after being sworn in last June. Shortly afterward, the mayor fired half of the 700-member police force for alleged links to drug traffickers.

Pimentel had promised to weed out corrupt officers and create a more professional police force, but months later he was still struggling to find qualified replacements.

His resignation comes amid a sharp increase in drug-related violence.

The corpses discovered Wednesday night had been shot and burned so badly they could not be identified, said Víctor Almanza, a detective for the Tamaulipas state police.

"We don´t even know if they are men or women," Almanza said.

The road the bodies were found on leads to an international bridge that links Nuevo Laredo with Laredo, Texas.

DEATH TOLL

So far this year, more than 57 people have been shot and killed in ambush-style attacks in Nuevo Laredo, compared to 23 during the same period last year.

Nuevo Laredo newspaper editors say they have not published the names of some victims of violence due to threats from drug traffickers.

The city of 330,000 across from Laredo, Texas, has been caught in a turf war between rival drug gangs fighting for billion-dollar smuggling routes into the United States.

In one of the latest attacks, four federal agents of a special operations and intelligence wing of the Federal Preventive Police were shot and killed last week by assailants who showered them with bullets in broad daylight in downtown Nuevo Laredo.

President Vicente Fox has assigned hundreds of federal agents armed with automatic weapons to patrol the city in an effort to curb drug violence, but the added police presence has done little to stop the killings.