(CNN) -- A car bomb killed at least four people in Ciudad Juarez, Mexico, authorities said.
It was the first time a car bomb has been used to attack federal police, said the city's mayor, Jose Reyes Ferriz.
The incident happened about 8 p.m. Thursday in the city's most violent zone.
Juarez municipal police spokesman Jacinto Seguro said Friday that federal police were responding to a call that a police officer had been killed.
"When they went to check the car, there was a dead body in there, dressed up like a police officer, but it wasn't one of ours," Seguro said. "They put him in a civilian car but dressed him up in a municipal police uniform. That's when the bomb went off. It's like an act of terrorism."
Two police officers, a paramedic and a civilian were killed, federal police spokesman Ramon Salinas said.
Six people, including a local news cameraman, were injured in the blast, authorities said. Four people -- three paramedics and a civilian -- were still being treated for injuries, police said. It was unclear how severe the injuries are.
Canal 5 video footage from moments after the blast captures seconds of confusion, followed by fires and debris scattered across the street. At one point, the cameraman shooting the video reaches for an injured civilian, and a police officer says, "Grab onto my arm. Grab onto my arm."
Although Mexican authorities say the attack was the result of a car bomb, a counterterrorism expert said 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 used by terrorist groups like Hezbollah, then this is a significant event, Burton said.
"The devil is in the details," he added.
Jose Marulanda, a security analyst based in Bogota, Colombia, called Friday's bombing a "turning point" in Mexico's drug war and found it remniscent of Colombia's own battle with cartels and car bombs in the 1990s.
"Because if they decide to start using car bombs one against the other ... then the whole society, bystanders, innocent people could be affected," he told CNN.
He said that while Friday's car bombing did not appear very sophisticated, he noted that Mexican drug cartels are often advised by former members of Colombian cartels. He said the Colombian cartels are influenced by FARC, the guerrilla group which has waged war against the Colombian government for decades and "has a lot of experience with explosives."
"We could expect more sophistication day after day if they decided to go on with these car bombs," he said.
According to Seguro, the Juarez cartel claimed the deadly incident in a graffiti message found in downtown Juarez.
In what appears to be black spray paint, the violent drug cartel wrote, "what happened on September avenue will keep happening to all the authorities who keep supporting El Chapo. Sincerely - the Juarez cartel."
The threatening message concluded: "We still have car bombs."
"This is significant because usually it's La Linea, the Juarez cartel's operatives, that sign the messages," Reyes said. "It's as if to say, 'Now, it's the big guys in charge, not the operatives.'"
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.
There had been relative calm in the city since elections were held there July 4.
But Thursday's explosion and an attack Sunday against Mexican federal police mark the third and fourth major incidents in recent weeks.
On June 29, a shooting between suspected drug traffickers and Mexican federal police left one officer dead. The shooting was seen as a watershed moment in the ongoing border drug war: Several bullets from that gunfight strayed across the border into Texas, hitting El Paso City Hall. There were no injuries reported on the U.S. side.
On April 24, six federal police officers were killed in a daylight shooting in Juarez. Hours later, a painted message found in the city, allegedly from members of La Linea, claimed responsibility for the attack. La Linea is an extension of the Juarez cartel, made up in part of former Juarez police officers, according to authorities.
Assaults against federal police have increased since they took full control of security in the city from the Mexican military on April 9.
"There have been at least a dozen, maybe 15, attacks against the federal police since we took over" security, Salinas said.
The Juarez cartel and the Sinaloa cartel have been in a bloody turf war since 2008. More than 5,000 people have been killed in drug-related violence in Juarez during the turf war, according to local authorities.
CNN's Nick Valencia and CNN en Espanol's Gustavo Valdes contributed to this story.