Simplify the following text for a Grade 9 native English-speaking person.
Original: According to Josephus however , it was Antonia , the mother of Livilla , who finally alerted Tiberius to the growing threat Sejanus posed ( possibly with information provided by Satrius Secundus ) , in a letter she dispatched to Capri in the care of her freedman Pallas .
Simplified: Exactly what caused his sudden downfall is unclear : According to Josephus , it was Antonia , the mother of Livilla , who finally alerted Tiberius to the growing threat Sejanus posed . She sent a letter to Capri in the care of her freedman Pallas .