Depends on the phone. I've recovered deleted texts on some older Motorola phones (e.g. V330) by using a program that dumps all the settings and user data. I have no idea how you would do it on a modern phone. Some phone OSes have better security than others. You'd need a guru to do this. Know any 15 year-old kids?

Hey, @lisanpat10​!

We'd love to help you recover your deleted texts, but as @drnewcomb2 mentioned, we need to know what type of phone you're using. 😊

Hey there! Just wanted to check in here and ask if you'd had any luck finding a way to retrieve your messages! Please let us know a little more about what kind of device you're using if you're still looking for help.

I understand if I did not backed-up before the accidental deletion the messages are gone locally. But how come the FBI can still get them during criminal investigation?

There always are possible records but it’s a PITA to find them. That’s why the FBI and other entities employ hackers.