Kate Upton -- FBI Probes to See If Hacker Hooked Her and 3 Others by Phishing

The leaked photos of Kate Upton and her boyfriend didn't get out because of an iCloud security breach ... but instead she may have been the victim of one of the oldest tricks in the online scamming game.

Law enforcement sources tell TMZ ... FBI investigators are looking into whether Kate and at least 3 other celebs -- Mary Elizabeth Winstead, Jennifer Lawrence, and Lea Michele -- were prime targets of phishing.

If you don't know, here's how it works ... you get an email informing you there's some sort of problem with your account, and in order to fix it you'll need to send personal info -- such as a username, phone number, credit card, or even a password. So, you unwittingly give the crooks all they need to jack your data.

We're told in this case ... the email sent to the celebs might have been doctored up to look like it was coming from Apple. Phishing's been going on since the early days of the Internet, but scammers are so sophisticated now ... their emails look identical to the real deal.

J-Law and Kate's pics -- some with her BF pitcher Justin Verlander -- were among the first ones released Sunday by the hacker.