The F.B.I. declined to comment.

There are many potential reasons Mexican officials, if they were serious about investigating, might want American help, even if they did not necessarily need it. The Mexican public is deeply suspicious of its government, so the participation of American law enforcement officials could help build trust in the findings.

But more than six months after the investigation was announced, some of the American concerns appear to be bearing out, according to victims of the spying and their lawyers, who have had access to the case files.

The government inquiry has failed to make headway in many basic areas, they contend. Prosecutors handling the case have yet to question any of the officials responsible for operating the surveillance technology, according to the victims’ lawyers and their review of the case file.

The Mexican government declined to offer specific comments on the investigation but said it remains “in the phase of exhausting different lines of investigation.” It also said that it had been in constant contact with the group of forensic analysts that first discovered the existence of the spyware, the Citizen Lab at the University of Toronto’s Munk School of Global Affairs, and that it had sought counsel from other national and international experts.

The Citizen Lab seemed taken aback by the assertion.

“That’s a surprising statement, given that we have had exactly one meeting with them and have received no further follow-up,” said John Scott-Railton, a senior researcher there. He said the Citizen Lab answered the government’s questions at that meeting, in October, and even suggested a list of evidence to preserve for the investigation.

The spying software, known as Pegasus, infiltrates smartphones to monitor every detail of a person’s cellular life — calls, texts, email, contacts and calendars. It can even use the microphone and camera on phones for surveillance, turning a target’s smartphone into a personal bug.