Mexico City's mayor said on Monday that no expense will be spared and no line of investigation ignored in the hunt for the killers of four women and a photojournalist who had fled the state where he worked after fearing for his safety.
The United Nations High Commission on Human Rights condemned the killings, saying that the bodies showed signs of torture and sexual violence and that a climate of impunity "is one of the obstacles to practicing freedom of expression in Mexico."
"We are all outraged by this crime," Mayor Miguel Angel Mancera said at a news conference.
"There will be no impunity in this matter. No line of investigation will be discarded."
Journalist protection groups have expressed fears that authorities won't consider the killing of Ruben Espinosa, 31, as related to his work, even though colleagues say he had fled the state of Veracruz due to threats related to his reporting.
The office of Mexico City's chief prosecutor said late on Monday in a statement that investigators had found a red Ford Mustang linked to one of the victims abandoned in a neighborhood to the south. They believe it belonged to a 29-year-old woman who they think is Colombian but have not identified yet.
The statement also said the building's security camera was damaged and did not contain video of the scene.
Prosecutor Rodolfo Rios Garza said on Sunday that authorities were following protocols for crimes against journalists and crimes against women as well as looking at robbery as a possible motive in the case.
But when dealing with killings of journalists authorities in Mexico have in the past often been quick to discard their work as a motive, though the country is the most dangerous in Latin America for reporters. Some 90 percent of murders of journalists in Mexico since 1992 have gone unpunished, according to the Committee to Protect Journalists.
"What's particularly pernicious is that violence against the press is violence against society," said Dario Ramirez, director of the free press advocacy group Article 19.
"There are many places in the country where silence paves the road so that organized crime, corruption, everything that destroys a society can continue in a manner without ... setbacks or obstacles."
Espinosa was buried on Monday after a private memorial service.
The city prosecutor's office said it was waiting on test results to confirm the reports that the victims were tortured and that some of the women may have been sexually assaulted.
An official from the prosecutor's office said three of the female victims had been identified, but a fourth - presumed to be Colombian - had not.
Rios gave the women's ages as 18, 29, 32 and 40. All were shot in the head with a 9 mm weapon. Espinosa sustained severe injuries to his face before he was killed, Ramirez said.
Rios also said the apartment was ransacked and robbed. Three of the women lived there and a fourth was the housekeeper. One woman was a friend of Espinosa's from Veracruz, where he had worked for eight years.
The bodies were found late on Friday in a building in a middle-class neighborhood. The building was in range of several security cameras on the street and Rios said officials have video evidence of the crime. The attackers would have had to go through two doors to get inside the building and neither showed signs of damage or a break-in.
Friends said Espinosa had fled Xalapa - the capital of Veracruz - to Mexico City in June after saying that unknown people were following him, taking his photograph and harassing him outside his home.
The coastal state is a particularly dangerous place for reporters, with 11 journalists being killed there since Govenor Javier Duarte took office in 2010. Two others, including Espinosa, have been killed outside the state and three have gone missing.
Duarte's office issued a statement on Monday saying the governor had called Mexico City's mayor to offer his support in the investigation and instructed Veracruz's top prosecutor to collaborate with his Mexico City counterpart. A day earlier he had called the killings "aberrant."
In June Duarte accused some reporters of being involved in organised crime.
"We all know who is involved in the underworld," Duarte said. "There's no reason to confuse freedom of expression with representing the expression of criminals via the media."
Fears that Espinosa's death could go unpunished were fueled by a news conference on Sunday in which Rios didn't acknowledge that Espinosa had sought refuge in Mexico's capital, saying he came to the city for "professional opportunities." The comment provoked shouts and protests from reporters.
Espinosa worked for the investigative newsmagazine Proceso and other media outlets
Article 19, an advocacy group, said Espinosa's death was a milestone in violence against the press because he was the first journalist to be killed in exile Mexico's capital. The agency said that in the last five years it has helped about 70 journalists under threat find refuge in the capital.