“That’s not O.K.,” Professor Kovera said. “You can’t alter the face of the suspect.”

Among the police departments that do sometimes touch up a suspect’s photo is the country’s largest, the New York Police Department, often as a last resort.

“All efforts are made to alter the filler photos, not the subject’s photo,” said Sgt. Mary Frances O’Donnell, a spokeswoman for the department. When confronted with a suspect whose scars or tattoos would stand out from the filler images, the department’s photo unit adds the same feature to the five filler photos “to ensure photo arrays are fair and impartial,” she said, adding that investigators document all changes.

Officials at other police departments, including in Los Angeles, Chicago and Philadelphia, said they keep their hands away from Photoshop.

“Adding or removing tattoos is not something we do,” said Detective Donny Moses, a spokesman for the Baltimore Police Department.

“We don’t tamper with them at all,” he said of photos for arrays. “We would get killed in the courts as well as the media. That’s something we don’t mess with.”

In some cases, doing nothing about distinguishing marks may hurt suspects the most. A state appeals court in New Jersey last year tossed out an identification in which a witness had selected a man as the perpetrator because he was the only person with face tattoos in a photo array.

In a study published in 2016, researchers in the United States and England asked witnesses to watch someone with a distinctive feature commit a crime. When researchers then produced a photo array in which only one person had the distinctive feature, witnesses were more likely to identify that person as the perpetrator — even when it was not the same person from the video.