A car bomb has killed at least three people in Ciudad Juarez, Mexico, the city's mayor, Jose Reyes Ferriz, said on Friday.

It was the first time a car bomb has been used to attack federal police, the mayor said. "The violence is escalating," he said.

Two police officers and a paramedic died and seven people, including a local news cameraman, were injured in the blast Thursday night. A female paramedic was in grave condition, he added.

The incident happened around 8 p.m. in the city's most violent zone.

Before the incident, a call to the city's emergency center reported "lots of shooting and killing," Reyes said. "When the federal police responded, there was a suspicious car there. One of them went to go check it out and when he opened the door, the bomb went off," he said.

While Mexican authorities say the attack was the result of a car bomb, a counter-terrorism expert told reporters there is some confusion about exactly what caused the car to explode.

"For this to be an improvised grenade attack, in some capacity, it doesn't surprise me," said Fred Burton, vice president of intelligence at Stratfor, a privately owned global Intelligence service.

But if this particular car bomb was manufactured to the level of sophistication similar to those terrorist groups like Hezbollah are using, then this is a significant new event, said Burton.

A claim of responsibility scribbled in a graffiti message was later found in downtown Juarez. It was purportedly signed by the Juarez drug cartel. "This is significant because usually it's La Linea, the Juarez cartel's operatives, that sign the messages," Reyes revealed.

"It's as if to say, 'Now it's the big guys in charge, not the operatives.'"

Federal police spokesman Ramon Salinas said the blast in the Mexican border city took place as authorities were responding to "some sort of emergency." Earlier in the day, police announced the arrest of Jesus Armando Acosta Guerrero, believed to be a leader in the Juarez cartel - one of two drug trafficking organizations operating in the area.