Which president's death is still a mystery?

Answer 1

Mystery is a rather vague term.

Warren Harding died suddenly and people wondered if he had been poisoned- his wife being the main suspect. In fact, a sensational book was written that suggested that she killed him. She refused to allow an autopsy to be performed so there is is still something of a mystery about the cause of death.

There was also much speculation about the death of Zachary Taylor- people suspected that he may have been poisoned with arsenic His body was exhumed in 1991 for tests but nothing suspicious was found.

There is still discussion about the deaths of Lincoln and Kennedy- not the cause, but who and why they were involved in the shootings.