And Microsoft co-founder Bill Gates has stepped into the ring, first being quoted as supporting the FBI's bid to force Apple to provide a backdoor into its mobile operating system, and later saying he had been misunderstood.
A report in the The Verge said 12 of these iPhones were listed by Apple in its response to a request from a district court in New York. The remaining device is mentioned in a letter from the US Department of Justice.
Apart from these 13, the district attorney of Manhattan district, Cyrus Vance Jr, told the US National Public Radio that his cyberlab had asked Apple to break into 175 iPhones.
The FBI obtained a court order last Tuesday (February 15) asking Apple to provide it with an updated version of iOS so that it could gain access to an iPhone 5c belonging to the San Bernardino Department of Health. The phone had been in use by Syed Rizwan Farook, one of two people involved in an act of terrorism in December last year in which 14 people were killed.
Of the 13 iPhones cited earlier, eight are said to be running versions of iOS 7.0 or earlier, thus making the obtaining of data from them relatively easy. There are also two iPhone 3s and one iPhone 4; both these models do not have the additional security features that have been incorporated into iOS since version 8.0. (corrected)
These features allow 10 guesses to be made as to the passcode; after five, the length of time before one can make the next guess increases, until it becomes an hour after the ninth guess. After 10 guesses, the phone becomes unusable. The FBI wants Apple to disable these two features in a modified version of iOS and load it onto the iPhone in the terrorism case, so that it can use brute force methods to guess the passcode.
The FBI has used a 227-year-old law, the All Writs Act in its pursuit of Apple; the law makes it compulsory for an individual or an organisation to assist officials in their investigations and is generally used only when there is no other way to obtain co-operation.
Gates made his arguments in favour of the FBI to the Financial Times in which he was quoted as saying: "This is a specific case where the government is asking for access to information. They are not asking for some general thing, they are asking for a particular case."
The Microsoft billionaire appeared to be unaware of the latest development in the case when he made this statement. He also seemed to be unaware that the Reform Government Surveillance organisation, of which Microsoft is a member, had issued a mild statement in support of Apple and Cook's statement.
Later in an interview with Bloomberg TV, Gates said he had been misunderstood. But what he had to say did not change to any marked degree his earlier statements.
Apple has until Friday (Saturday AEDT) to respond to the FBI's court order. Former US solicitor-general Ted Olson will represent the company in court.