They have now stated multiple times that there is a line of phones for this. And they aren't asking for a "key". They are asking Apple to rewrite the OS completely so that it's easier for the FBI to hack them. Their official request is 1) Remove delay in password, remove auto erase, rewrite the entire touch screen firmware so that external software can access it and make touches without actually touching the screen (e.g. so they can run software to brute force the password).
Do you think Apple should open up a brand new FBI–hack–into–phone–department? 10 devs that will create firmware for each phone request. Or is FBI asking that they create a generic piece of software that they will get on a secure hard drive to use in their labs (see the congressional hearing, this is what Comey is asking for–it's like 3 hours long, but if you are passionate about the subject, you'll actually enjoy it; the questions asked are good)?