Facebook founder Mark Zuckerberg has sided with fellow tech-giant Apple in the battle to have the phone of a San Bernardino shooter unlocked.

The FBI had ordered Apple to disable the security software on the phone, once-owned by one of the dead shooters, Syed Farook, but the Silicon Valley firm has relented.

“I don’t think that requiring back doors to encryption is either going to be an effective thing to increase security or is really the right thing to do,” Zuckerberg said, according to BBC.

“We are pretty sympathetic to Tim [Cook] and Apple.”

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The social networking site had earlier released a statement saying a ruling forcing Apple to access the phone – through a “back door” or new anti-encryption software – could set a dangerous precedent. 

Meanwhile, victims of the San Bernardino attack are set to file a legal brief in support of the US Government’s attempt to force Apple to unlock the encrypted iPhone, a lawyer representing the victims says.

Stephen Larson, a former federal judge who is now in private practice, said that the victims he represents have an interest in the information, which goes beyond the Justice Department’s criminal investigation.

“They were targeted by terrorists, and they need to know why, how this could happen,” Mr Larson said.

Fourteen people died and 22 others were wounded in the shooting attack on December 2, 2015 by a married couple who were inspired by Islamic State militants and died in a gun battle with police.

Apple claimed that cooperating with the FBI probe would undermine overall security for its devices.

Entry into the fray by victims gives the federal government a powerful ally in its fight against Apple, which has cast itself as trying to protect public privacy from government overreach.

Mr Larson said he was contacted a week ago by the Justice Department and local prosecutors about representing the victims, prior to the dispute becoming public.

He declined to say how many victims he represents, although he said he is representing them for free, and will file an amicus brief in court by early March.

An Apple spokesman declined to comment, but the company so far has pushed back, arguing that such a move would set a dangerous precedent and threaten customer security.

FBI director James Comey, a long-time critic of digital encryption, wrote in a statement on the FBI’s website: “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.”