According to "Patrology" by Johannes Quasten, (Christian Classics, 1950), the letter from the Smyrna Church to the Christians in Philomelium is the oldest detailed account extant of the martyrdom of a single individual. Thus, it is often regarded as the first "Acts of the Martyrs."
Polycarp's martyrdom occurred in A.D. 156, the year after he had returned from Rome where he had been called by Pope Anicetus to discuss several church matters.