The FBI have sent a request to Apple asking for their help to unlock the passcode protected iPhones owned by Mohammed Saeed Alshamrani who carried out a mass shooting in the US.
The shooting took place at the Florida naval air Station last month and the phones used by the shooter may have information on them, although they are protected by passcodes.
Apple has had this in the past and they have been unable to unlock these devices, they have made it clear that there is no back door in iOS that can be used to unlock these type of devices. You can see a statement from them below on this new case.
We have the greatest respect for law enforcement and have always worked cooperatively to help in their investigations. When the FBI requested information from us relating to this case a month ago, we gave them all of the data in our possession and we will continue to support them with the data we have available.
If apple did install a backdoor in their OS to be used by law enforcement, it could also be used by hackers to obtain peoples data, this is one of the reasons why it has not been done.