CIUDAD JUAREZ, Mexico (AP) — Mexican investigators ran forensic tests to determine whether drug gangs used a car bomb in an attack on police patrol trucks that killed two officers the border city of Ciudad Juarez.

Attorney General Arturo Chavez said the assailants may have packed the car with explosive material, or they may have launched grenades at the trucks. He said the test results were expected later Friday.

So far, Chavez said at a news conference in Mexico City, "we have no evidence anywhere in the country of narco-terrorism."

A car bomb would mark an unprecedented escalation of Mexico's drug war and confirm long-standing fears that the cartels are turning to explosives in their fight against security forces. It would be another alarming sign of the growing boldness and power of drug traffickers who attack police and soldiers daily and recently assassinated a gubernatorial candidate a week before state elections.

The car rammed two police trucks at a major intersection in Ciudad Juarez on Thursday evening, killing a federal police officer, a municipal policeman and a medical technician, according to a federal police statement. At least seven officers and two civilians were wounded, state police said.

The identities of the civilians were unclear, and the fate of the driver of the attack vehicle was unknown.

What appeared to be the charred bottom half of the attacking car still lay at the scene Friday. The debris from the blast was spread out over a 300-yard (300-meter) radius. The explosion also blew out the windows of a nearby home and blackened the corner of the building nearest to the crash.

A masked federal police officer there said the size of the blast suggested to him that the car had been loaded with explosive material. The officer, who spoke on condition of anonymity because he was not authorized to give details about the investigation, said he didn't believe grenades would cause such widespread destruction.

"Thank God we weren't home," said a woman who lives in the damaged house. She refused to give her name, citing safety reasons, before driving away from the scene Thursday.

Federal police said in a statement that the attack was in retaliation for the arrest of a top leader of La Linea drug gang, Jesus Acosta Guerrero, earlier in the day.

Police said Acosta Guerrero, 35, was the "operations leader" of La Linea, which works for the Juarez drug cartel. He was responsible for at least 25 killings, mainly of rival gang members, and also ordered attacks on police, the statement said.

Drug gangs have previously attacked Mexican soldiers and law enforcement officers with grenades and powerful rifles, but seldom have been known to use explosives.

However, there have long been fears that Mexican cartels might turn to bombings. Soldiers have seized homemade explosives from gang vehicles after gunbattles, and assailants have stolen explosive material from transport vehicles.

Attacks on security forces and government officials have become increasingly bold since Felipe Calderon deployed thousands of federal troops and police to drug trafficking hotspots in December 2006.

A carefully planned ambush last month killed 12 federal police officers in the western state of Michoacan. In the northeastern states of Nuevo Leon and Tamaulipas, drug gangs have thrown up roadblocks to keep soldiers from coming to the aid of colleagues under attacks. And a week before July 4 local and state elections, suspected drug cartel members ambushed and killed the leading candidate for governor of Tamaulipas.

On Wednesday, gunmen killed the nephew of the Chihuahua's governor-elect, although it was unclear who was behind the attack. Mario Medina, nephew of Cesar Duarte, was shot in the back as he tried to escape from would-be kidnappers in the state capital, also named Chihuahua, prosecutors' spokesman Eduardo Esparza said.

Chavez said at least 24,826 people have been killed in drug gang violence since Calderon launched his military-led offensive. He reiterated the government's argument that violence has surged in large part because cartels are splintered and on the defensive.

"The actions of the government are forcing criminals to modify their strategies," he said. "The strongest keep what they have, while the defeated are looking for new turf, and this means invading territories that belonged to someone else. This provokes confrontations and internal wars."

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E. Eduardo Castillo reported from Mexico City.