Mr Duggan, 29, was shot dead by police in Tottenham last August. It was initially claimed he had opened fire on officers but the IPCC admitted at a pre-inquest hearing that this was a mistake.
A peaceful protest that followed his death turned into the ugly scenes of rioting and looting that spread across the country.
But IPCC deputy Chairman Deborah Glass indicated there may not be an inquest into Mr Duggan's death because it was a "breach of the law" to explain why some of the sensitive police evidence could not be released.
Mr Duggan's family criticised the IPCC, describing the watchdog as an "obstacle" to getting the truth from the police.
Carole Duggan, Mr Duggan's aunt, told the Today programme: "We're as in the dark now as we were in the beginning.
"We as a family believe that Mark was executed on the streets of London by the Metropolitan Police. That's all we do know really because all the information is being withheld from us."
Miss Duggan said there had been suggestions an inquest or public inquiry might be held in private.
"I would prefer to know the truth whether it is good or bad," she added. "If Mark was doing something wrong, we need to know because he was killed by the police; we don't know why, we don't know what he had done, if he had done anything at all."
Lord Carlile, a former independent reviewer of terrorism legislation, told the programme he backed the IPCC's call for a change in the law to address section 17.
"Section 17 is important but it does prevent the truth being made public in some cases," he said.
"My view is that Parliament and the Government should re-examine section 17 with a view to rebalancing it, preferably under judicial control."
He said the section that restricts intercepted evidence being disclosed to a coroner was designed to protect national security.
He added: "My suggestion would be if the IPCC wished to make such information public, an application should be made to an experienced senior circuit judge who day by day deals with evidential matters and an assessment could be made on the proportionality between keeping the information secret and releasing it to the public."
Intercepted phone evidence is the only kind of evidence to which the Ripa section applies. Intercepted emails and evidence gathered from bugging can be disclosed.
Lawyer Matthew Ryder, who acted for the Tomlinson family in the inquest into the death of Ian Tomlinson, who died after he was struck by a police officer during the G20 protests, said the Government should review the law.
"When you listen to the Duggan family, it is absolutely clear that it is vital there is an inquest and it's equally critical that the public is told why there isn't to be one," he said.
"There needs to be an urgent review of this provision. We're the only country in the world that uses this rule. It prevents potentially very important prosecutions and it seems to be at risk of preventing one of the most important inquests we could have at the moment."
He added there was "deference" shown to the police and security services over sensitive information.
"It's important for everybody because Mark Duggan's family and the local community need to know, the wider public need to know and the police need to be able to explain to people what they did and why," he said.