The Mexican attorney general's office said it would ask the US Federal Bureau of Investigation for help probing allegations that the government spied on journalists and activists.
Mexican President Enrique Pena Nieto has been under pressure to launch an independent investigation since charges were filed last week by a group of leading reporters, human rights activists and anti-corruption campaigners claiming that his government used highly invasive spyware against them.
The government denies the software -- an Israeli program called Pegasus -- was used on the plaintiffs.
Mexico's special prosecutor for crimes against journalists, Ricardo Sanchez, announced the creation of a "technical support group" to aid the investigation, with input from the FBI, the UN's International Telecommunications Union and other specialists.
"Cases of alleged spying on people in the public eye have occurred recently in other countries, and we are acting based on that international experience," he said.
But alleged victims rejected the announcement.
Prosecutors "have not clarified what type of technical assistance they will request," said the Miguel Agustin Pro Juarez Human Rights Center, whose executive director is among the alleged spying victims.
"This does not meet our demand for the establishment of an independent panel of experts," said a statement from the center, which investigated the abduction and suspected massacre of 43 students in the southern state of Guerrero in 2014.
The plaintiffs accuse Pena Nieto of planning a whitewash, and even threatening them when he reacted to their allegations by calling for people who made "false reports" to be punished.
Made by a secretive Israeli firm called NSO Group, Pegasus effectively turns a target's cell phone into a pocket spy, accessing the user's communications, camera and microphone.
According to The New York Times, which broke the story a week ago, at least three Mexican federal agencies have purchased some $80 million of spyware from NSO Group since 2011.
The company, which claims it only sells Pegasus to governments, says it has an agreement with clients that the software be used exclusively on people it identifies as suspected terrorists and criminals.