Relatives of some of the victims of the San Bernardino attack on Thursday backed the FBI's bid to force Apple to unlock an iPhone used by one of the assailants.

In a legal brief filed in the federal court in California where the case is being heard, they wrote that Apple wants to portray the debate as "one in which the privacy interests of millions of Americans are at stake in order to obtain sympathy for its cause."

"What is implicated here is the United States' ability to obtain and execute a valid warrant to search one phone used by a terrorist who committed mass atrocities," the 41-page amicus brief filed by attorney Stephen Larson said.

The brief includes a letter to Apple's CEO Tim Cook from Mark Sandefur, whose son was among the 14 people killed in the December 2 attack in San Bernardino, California.

"Recovery of information from the iPhone in question may not lead to anything new," Sandefur wrote.

"But, what if there is evidence pointing to a third shooter? What if it leads to an unknown terrorist cell? What if others are attacked, and you and I did nothing to prevent it?"

The legal document was filed as several tech companies and activists submitted their own court papers on Thursday supporting Apple's argument that unlocking the iPhone would set a dangerous precedent and jeopordize the encrypted data of customers across the globe.

The FBI has sought Apple's assistance in unlocking the iPhone used by Syed Farook, who was behind the San Bernardino massacre along with his wife Tashfeen Malik.

The agency has argued that by introducing encryption that can lock data, making it accessible only to the user, Apple and other tech companies are essentially creating "warrant-proof zones" for criminals and others that will cripple law enforcement and jeapordize public safety.

The brief filed by relatives of the victims said Apple was "disingenuous" in its arguments and was walking a "fine line when it equates the United States' efforts to legally search a terrorist's phone with indiscriminate hacking."

"The public, and the victims of this crime, have a strong right of interest in the United States' investigation and Apple's reasonable assistance in the investigation is warranted," it added.

"Broader questions about the fate of smartphone encryption and date privacy can be saved for another day and another forum."