“But, Adams further claimed, he had forgotten the password he had just set, and thus was unable to provide the FBI with a password that would unlock the phone.”
You know, the same thing happens to me every time the FBI takes my phones.
“Adams claimed that after he learned about the investigation into his conduct, he changed the password,” and he “increased the complexity of his password from four digits to six,” the document reads. Adams said he did this to stop members of his staff “from inadvertently or intentionally deleting the contents of his phone because, according to Adams, he wished to preserve the contents of his phone due to the investigation.”
According to reporting, they have actually done this before.
Most of the discussion I've read about this is along the lines of: clone the drive on the phone and then brute force the clones (circumventing the lockout issue: if you get locked out just keep trying on a new clone).
The FBI didn’t crack it. It was done by Cellebrite. I worked for Apple during the San Bernardino case. The suspect had an iPhone 5c, a model that did not have a Secure Enclave, and was vulnerable to brute force hacking through the Lightning Port. The Secure Enclave was released with the iPhone 5s, along with Touch ID, preventing peripheral access through the Lightning Port after the iPhone has been restarted.