“You are not just hacking anyone’s phone, you are hacking the phone of someone who has been granted immunity,” said Francisco Cox, one of the investigators and a prominent Chilean lawyer. “They couldn’t even search my bags in the airport.”

“If this can happen to an independent body that has immunity and that is invited by the government, it is a bit scary to think of what could happen to a common citizen in Mexico,” he said.

Since 2011, Mexico has purchased at least $80 million worth of the spyware, which is sold exclusively to governments, and only on the condition that it be used against terrorists and criminals. But an investigation by The New York Times and forensic cyberanalysts in recent weeks determined that the software had been used against some of the country’s most influential academics, lawyers, journalists and their family members, including a teenage boy.

The government has denied responsibility for the espionage, adding that there is no ironclad proof because the spyware does not leave behind the hacker’s individual fingerprints. It has promised a thorough investigation, vowing to call on specialists from the United Nations and the F.B.I. for help. One of the surveillance targets, the forensic analysis showed, was a United States lawyer representing victims of sexual assault by the Mexican police.

But the United States ambassador to Mexico, Roberta S. Jacobson, said the United States was not involved in the investigation. Opposition lawmakers and international officials are now calling for an independent inquiry into the spying scandal, declaring Mexico unfit to investigate itself.

“This case just on its face — and presuming the veracity of the allegations — is serious enough to warrant the creation of an international commission,” said James L. Cavallaro, a commissioner on the Inter-American Commission on Human Rights, which appointed the group of experts. “The commission shares the concerns of others: How can the government be trusted to investigate its own alleged violation of citizen rights given its track record in this matter?”