The family of Fusilier Lee Rigby, who was murdered by extremists in 2013, has criticised Apple for opposing a court order.
On Wednesday, Apple said modifying its software to help the FBI access data on San Bernardino gunman Syed Rizwan Farook's device would be "dangerous".
Mr Rigby's uncle Ray McClure said the company was "protecting a murderer's privacy at the cost of public safety".
But a number of tech leaders and privacy advocates have backed Apple.
They include Google's chief executive Sundar Pichai who tweeted that requiring companies to enable the hacking of their customer devices "could be a troubling precedent".
Whatsapp's founder Jan Koum added that he admired Apple's chief executive Tim Cook, and added: "Today our freedom and our liberty is at stake."
Mr McClure, who contacted the BBC after reading Apple's statement, said he thought Apple's approach was "short-sighted".
In December, Syed Rizwan Farook and his wife Tashfeen Malik were shot after killing 14 people in California.
US officials said Malik pledged allegiance to the so-called Islamic State in a social media post on the day of the shooting.
Apple had been ordered to help the FBI circumvent security software on Farook's iPhone, which the FBI said contained crucial information.
But the company said it would oppose the court order because it "set a dangerous precedent".
"Valuable evidence is on that smartphone and Apple is denying the FBI access to that information," Mr McClure told the BBC.
"If a court issued a warrant in the UK or United States to search somebody's house, you wouldn't stop them, you would allow them in - why should a smartphone be any different?
"If Mr Cook has no sympathy for terrorists, why is he stopping the FBI accessing those phone records?"
Apple has not indicated whether fulfilling the court order would be physically possible.
In an open letter on Wednesday, Tim Cook said Apple did cooperate with law enforcement.
"When the FBI has requested data that's in our possession, we have provided it," he wrote.
"We have also made Apple engineers available to advise the FBI... we have worked hard to support the government's efforts to solve this horrible crime. We have no sympathy for terrorists."
The company said it would oppose the court order because it would create a "backdoor" in the security of its operating system.
"In the wrong hands, this software — which does not exist today — would have the potential to unlock any iPhone in someone's physical possession," the company said.
But Mr McClure said companies were forgetting about the victims of crime, and the families of those killed.
"I would hate to see on the streets of London another murder like happened to Lee Rigby, I'd hate to see another attack like happened in Paris," he said.
"How many victims of crime are not getting justice because of Apple's stance?"
However, privacy advocates have backed Apple's position.
"The FBI's request, which would in practice require Apple to rewrite its operating system to weaken security protections, would set a very dangerous precedent," said Sherif Elsayed-Ali from Amnesty International.
"Such backdoors undermine everyone's security and threaten our right to privacy.
"This isn't an issue about reading a known terrorist's texts - it's about how the functionality could be abused in other ways."
Greg Nojeim from the Centre for Democracy and Technology agreed: "If the order stands, Apple and other technology companies could be ordered to build backdoors - essentially defects - into other devices, rendering them insecure and vulnerable to attack by law enforcement and by others as well."
In its statement, Apple said: "The government is asking Apple to hack our own users and undermine decades of security advancements that protect our customers.
"We can find no precedent for an American company being forced to expose its customers to a greater risk of attack."
Mr McClure said he appreciated the value of encryption but that some companies had taken things too far.
"I'm not saying take encryption off the iPhone, but there has to be a balance," he told the BBC.
"Where there is a legally obtained warrant for information we should be helping the authorities get that information.
"That can also clear an innocent person too, it works both ways."