A Los Angeles attorney, Stephen Larson, said he represents at least several families of victims and other employees affected by the attack. He said the U.S. Attorney in the case, Eileen Decker, sought his help.
Larson said he will file a brief supporting the Justice Department before March 3.
The victims "have questions that go simply beyond the criminal investigation ... In terms of why this happened, how this happened, why they were targeted, is there anything about them on the iPhone - things that are more of a personal victim" view, Larson said.
Magistrate Judge Sheri Pym ordered Apple last week to assist investigators by creating specialized software that would let the FBI rapidly test random passcode combinations to try to unlock the iPhone and view data stored on it.
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The county-issued iPhone 5C was used by Syed Farook, who with his wife, Tashfeen Malik, killed 14 people at an office Christmas party in December before they died in a gun battle with police in San Bernardino. The government said they had been at least partly inspired by the Islamic State.
Robert Velasco, whose 27-year-old daughter Yvette Velasco was killed in the shooting, told The Associated Press that he didn't have to think long before agreeing to have his name added to the legal filing in support of the FBI.
"It is important to me to have my name in there," Velasco said. "I lost my daughter in this and I want the court to see that I am seeking justice for my daughter."
Velasco said the phone could reveal other terror plots or that other people were involved in planning the San Bernardino attack.