Nursing home workers across the country are posting embarrassing and dehumanizing photos of elderly residents on social media networks such as Snapchat, violating their privacy, dignity and, sometimes, the law.

ProPublica has identified 35 instances since 2012 in which workers at nursing homes and assisted-living centers have surreptitiously shared photos or videos of residents, some of whom were partly or completely naked. At least 16 cases involved Snapchat, a social media service in which photos appear for a few seconds and then disappear with no lasting record.

Some have led to criminal charges, but most have not, even though posting patients’ photos without their permission may violate the Health Insurance Portability and Accountability Act, the federal patient privacy law that carries civil and criminal penalties.

The incidents illustrate the emerging threat that social media poses to patient privacy and, at the same time, its powerful potential for capturing transgressions that previously might have gone unrecorded. Abusive treatment is not new at nursing homes. But the posting of explicit photos is a new type of mistreatment — one that sometimes leaves its own digital trail.

In February 2014, a nursing assistant at Prestige Post-Acute and Rehab Center in Centralia, Wash., sent a co-worker a Snapchat video of a resident sitting on a bedside portable toilet with her pants below her knees while laughing and singing.

The following month, a nursing home assistant at Rosewood Care Center in St. Charles, Ill., recorded another using a nylon strap to lightly slap the face of a 97-year-old woman with dementia. On the video, the woman could be heard crying out, “Don’t! Don’t!” as she was being struck. The employees laughed.

PrestigeCare said it fired the employee, alerted authorities and instituted stricter cellphone and social media policies. “We take these situations very seriously and are thankful that our own internal procedures alerted us so promptly to the issue.”

ProPublica identified incidents by searching government inspection reports, court cases and media reports. Marian Ryan, the district attorney of Middlesex County, Mass., whose office is pursuing elder abuse charges, said she suspects such incidents are underreported, in part because many of the victims have dementia and do not realize what has happened.

The federal agency charged with enforcing the privacy law, the Office for Civil Rights in the Department of Health and Human Services, has not penalized any nursing homes for violations involving social media or issued any recommendations to health providers on the topic.