The FBI recently said that it failed to access data on 7,000 encrypted mobile devices in its possession, describing the predicament as a "huge, huge problem" for law enforcement.
Technology companies insist that strong security based on data-scrambling encryption is essential to protecting digital privacy.
The FBI demanded that Apple help it disable the locking program, which Apple refused to do on the grounds that creating software to do so would result in something that could potentially unlock any iPhone.
"It actually highlights an issue that you've all heard about before with advance of the phones and the technology and the encryption, law enforcement, whether it's at the state, local or the federal level, is increasingly not able to get into these phones", Combs said at a press conference on Tuesday.
FBI Director Christopher Wray said last month that federal agents were still seeking access to 6,900 mobile devices.
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But iPhones locked with a fingerprint ask for the user's pass code after 48 hours if they have not been unlocked by then.
The agency has not said what make or model of phone they are not able to access, and this is information that will not be revealed - and for very good reason, says the Federal Bureau of Investigation. If Kelley had used a fingerprint to lock his iPhone, Apple could have told officials they could use the dead man's finger to unlock his device, so long as it had not been powered off and restarted.
It is still unclear what type of phone the Texas shooter used.
The two sides have sparred in the past over the first investigative steps involving an iPhone.
This incident echoes the San Bernadino shooting in 2015. That foreclosed the possibility of an automatic backup to Apple iCloud servers, which could have been accessed by investigators.
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Combs' comments suggest that critical clues to Kelley's motivations or potential co-conspirators remain inaccessible on his device without knowing the password.
But that impulse may be tempered by another consideration: So far, the Texas gunman appears to have acted alone in what some officials have called a domestic violence problem that escalated into a mass murder.
The Trump Justice Department says it will take a tougher approach toward getting information but hasn't said how. "Public shaming didn't work with Apple".
While FBI Director Christopher Wray has warned there are almost 7,000 phones that can not be opened, and those technologies are making it harder to fight terrorism and crime, Congress has shown little interest so far in tackling the issue.
"This failure to get encrypted information in a timely manner causes law enforcement to waste even more valuable time and resources", Sessions said in NY on Thursday.
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