Can't be mercury, since it's liquid at room temperature and so wouldn't form "fibres". Americum . . . wouldn't be impossible (and it's still used in smoke detectors to this day, I believe), but the amount in a stack of smoke detectors isn't quite the worst case—there would be more radioactive material in an orphaned radiotherapy or radiography source, which is also wildly improbable but not quite impossible as a multivitamin additive. At least it isn't likely to be an abandoned Soviet radioisotope generator this time.