In a new statement FBI Director James Comey said the demand was "about the victims and justice". "They were targeted by terrorists, and they need to know why, how this could happen," said Stephen Larson.
He declined to say how many of the victims he was representing, but did add that he would not be charging them a fee.
`Justice`
Fourteen people were killed and 22 injured when gunman Syed Rizwan Farook and his wife Tashfeen Malik opened fire in California last December.
However, Apple chief executive Tim Cook has described the FBI`s order as "dangerous", "chilling" and "unprecedented".
He has said the firm would have to build a new operating system in order to comply.
"We strongly believe the only way to guarantee that such a powerful tool isn`t abused and doesn`t fall into the wrong hands is to never create it," the firm states in a Q&A on the Apple website.
Facebook and Google have voiced support for Apple in the dispute.
"We simply want the chance, with a search warrant, to try to guess the terrorist`s passcode without the phone essentially self-destructing and without it taking a decade to guess correctly," said the FBI in its statement.
"That`s it. We don`t want to break anyone`s encryption or set a master key loose on the land.
"Maybe the phone holds the clue to finding more terrorists. Maybe it doesn`t. But we can`t look the survivors in the eye, or ourselves in the mirror, if we don`t follow this lead."
Last week anti-virus creator John McAfee offered to unlock the iPhone for them.
"It will take us three weeks," he told Business Insider, adding that he would eat his shoe on television if his team failed.
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