FBI extracted her deleted Signal messages from a suspect's iPhone.

Signal is end-to-end encrypted, with no plaintext on the server; once deleted, it's gone.
But the FBI found snapshots of each push notification in iOS's notification database—every time a message pops up on the lock screen, the iPhone stores a snapshot internally, including the plaintext content.
Users think they've deleted the conversation, but their phone keeps a backup in the notification history.
This is the first related prosecution after Trump designated Antifa as a terrorist organization; 404 Media reported on this evidence collection method disclosed during the trial testimony. End-to-end encryption protects the transmission, but devices have many "legitimate" caching mechanisms—notifications, backups, system logs—each potentially serving as a backdoor.
Encryption software safeguards you from eavesdropping. But when your phone helps you "remember," it also helps others record.