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 partially 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, including a case filed earlier this month in California against a nursing assistant. 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.