Bold for emphasis.
As has been stated by others here as well as Cook himself, Apple has no master key to open the 'box' for the police (read: open the iPhone). But that also in itself isn't the issue.
The issue is that the FBI wants Apple to CREATE THE MEANS that can be used, so that a master key would not be necessary; something which Apple can not inherently do. Not just because it would take a hell of a long time to crack the encryption, let alone opens the door for those with criminal intent to have access to exposed secure data, but opens Apple up to a flurry of liability lawsuits should this program fall into the wrong hands. That puts Apple as a company in jeopardy, let alone the personal data of every person using an Apple product.
And to be honest, the time it would take to create said means would be much longer than brute forcing a 4-digit passcode (10000 chances), 6-digit passcode (1,000,000 chances), or if Farook modified it, or alphanumeric (infinite).
In short, the FBI is being lazy.
BL.