Here's why St. Cloud mall attacker's iPhone may keep its secrets

Authorities trying to explain what led Dahir Adan to stab 10 people at a St. Cloud mall last month hope Adan's iPhone will yield some clues. For now, though, the iPhone isn't talking.

It's locked, and FBI investigators are now assessing their "legal and technical options" to gain access, FBI Special Agent Richard Thornton told reporters Thursday.

Breaking in, though, could prove very difficult, as the FBI found out in its investigation of a mass shooting last December in San Bernardino, Calif., said Andy Greenberg, a senior writer at Wired magazine.

iPhones have a feature that counts up each time the wrong passcode is entered, and after 10 tries the phone's memory is wiped clean, he said. The FBI tried to force Apple to find a way around the firewall in the San Bernardino case, which involved an iPhone 5c.

The agency eventually paid for the phone to be hacked when the company refused. Hackers have since found a software flaw that allowed the FBI access to the phone's memory.

"There may be other ways of breaking into an iPhone 5c, and really we don't care that much about the iPhone 5c except for the fact that the San Bernardino shooter had one," Greenberg said. "We care a lot more about hacking techniques that could get us into modern iPhones."

Authorities haven't said what generation of iPhone Adan was using before he the Sept. 17 attack, but Greenberg said the iPhone 6 and 7 models have even stronger and more sophisticated encryption than what the FBI faced in San Bernardino.

That includes Secure Enclave, a new feature in iPhone security in which fingerprint data is siphoned off and stored in an encrypted form in a separate part of the phone, and can only be decrypted with a key by the Secure Enclave.

"That makes it really hard to change the memory of the phone, even if there's physical access to it," Greenberg added.