Apple has long advertised iPhones as infallible lockboxes that nobody can break into. They are, without a doubt, one of the most secure consumer devices on the market. Even the FBI has a hard time cracking one, which is why the agency has formally requested Apple to help crack two iPhones.
The iPhones in question belong to a person who killed three people in a mass shooting last month. Although the FBI has access to the devices, it is unable to get past the lock screen. Repeated attempts to guess the passcode has yielded no results. One gets no more than ten attempts to input the correct code before the device automatically factory resets itself, so brute-forcing through it is not an option.
Furthermore, the FBI has also solicited help from 'third parties' that claim to have expertise in bypassing iPhone security. Nobody seems to have had any luck cracking the devices, though. It could be why the FBI decided to approach Apple directly. The gunman also fired a bullet at one of the devices, making it even harder to investigate. The exact models of the iPhones in question are unknown, but it is reasonable to assume that they're relatively new.
Apple says that it is willing to work with the FBI and has handed over all data it has about the shooter. That, however, is just unencrypted data harvested off iCloud and will do little in the way of helping investigators unlock the phones. Apple stands by its claim that even it cannot unlock an encrypted iPhone.
Back in 2016, the FBI was faced with a similar predicament with another mass shooter. It resulted in a very public spat between Apple and the FBI that ended in a courtroom. Ironically, the iPhone 5c in question was eventually cracked by an external actor and not Apple. It lacked key security features such as Touch ID and Secure Enclave, so unlocking it must have been a lot easier.