Army veteran William Penner used to jokingly call the thick yellow crust that crept across his young son Matthew’s scalp “Agent Orange” after the toxic defoliant sprayed on him in Vietnam before the boy was born. The joke turned sour a few years ago, when Matthew, now 43, was diagnosed with a host of serious illnesses, including heart disease, fibromyalgia and arthritis.

Similar worries struck vet Mike Blackledge when staffers at a local Veterans Affairs hospital suggested his children’s diseases could be linked to his time in Vietnam. His son has inflammatory bowel disease so advanced he wears a pouch to collect his waste, and his youngest daughter has neuropathy, spinal problems and gastrointestinal issues. His oldest daughter — the one born before he went to fight in Vietnam — is fine.

They, like thousands of others, are grappling with a chilling prospect: Could Agent Orange, the herbicide linked to health problems in Vietnam veterans, have also harmed their children?

For decades, the Department of Veterans Affairs has collected — and ignored — reams of information that could have helped answer that question, an investigation by ProPublica and The Virginian-Pilot has found.