Mexico City (CNN) -- What happened to 12 youths who were kidnapped in broad daylight from a bar in Mexico?
The mystery, which has jolted the country's capital for months, appeared to be one step closer to resolution Monday:
The victims were killed shortly after the kidnapping, Mexico City Prosecutor Rodolfo Rios Garza told reporters on Monday. And the motive was revenge, he said.
The crime occurred in retaliation for another killing at a bar in the city's upscale Condesa neighborhood, he said.
The prosecutor's revelation comes a day after an attorney representing victims' families said authorities had found that at least 10 of the bodies discovered last week in a shallow grave near Mexico City corresponded to the group kidnapped from the Heaven after-hours club.
The mysterious mass kidnapping in Mexico's capital in May drew international attention after furious family members protested, claiming authorities weren't taking the case seriously.
The allegations have sparked concerns about whether violence has surged into Mexico's capital, where residents largely have been spared from drug war casualties seen in other parts of the country.
Prosecutor: 'We are not talking about any cartel'
Rios said Monday that the group behind the kidnapping was La Union de Insurgentes, which has been tied to drug distribution in the capital. But he dismissed concerns about larger forces at work.
"We are not talking about any cartel," he said.
But even with the new information released by authorities, victims' family members said their outrage over the case hasn't dimmed.
One stood in the doorway of Monday's press conference, wearing a T-shirt emblazoned with all 12 victims' faces.
As officials tried to get her to move, Julieta Gonzalez told them, "I am not moving from here. I am not going to do anything. I only want to hear."
Gonzalez -- whose daughter, Jennifer Robles, was among those taken from the bar -- said she wanted to listen because authorities have consistently given reporters details about the case before telling family members.
Seven suspects arrested
Mexico's Attorney General's Office said Sunday that at least 10 of the remains found correspond to the kidnapped youths, attorney Ricardo Martinez told CNNMexico. Officials have not provided details about three other bodies found in the grave.
Authorities found the grave last week in the town of Tlalmanalco, in the state of Mexico, about 30 miles east of Mexico City, the state-run Notimex news agency reported. Investigators haven't said what led them to the grave.
Authorities have arrested seven suspects in connection with the crime, Rios said Monday.
The owner of the bar, a co-owner and two bar employees are among those who have been arrested in the case.
A suspect detained at a hotel in Mexico City last week told authorities he and others had taken the victims to Tlalmanalco, Rios said.
"We will not stop until we have achieved the detention of the rest of the participants in these events," he said, describing what he said had been an exhaustive investigation.
Families call for independent investigation
But the mother of at least one of the victims said she was far from satisfied with the investigation so far, and wants more proof about what investigators exhumed from the grave last week.
"No one is sure of what is there. We have not seen clothes. They have not shown us bodies. They have not shown us anything," mother Leticia Ponce told CNN en Español Monday.
Victims' family members have said they don't trust Mexican authorities and have called for an independent international investigation of the case.
"I have always distrusted them. ... They have not shown me a hair or a nail from my daughter. I am not taking anything until they prove to me that it is my daughter," Gonzalez said. "They are staging it."
CNNMexico's Veronica Diaz Favela and Arturo Ascencion, CNN's Catherine E. Shoichet, journalist Nick Parker and CNN en Español's Mario Gonzalez contributed to this report.