The FBI investigation into the failed July 13 assassination attempt on former President Donald Trump has revived the argument over whether Big Tech firms should provide law enforcement with a backdoor method to break their own "unbreakable" encryption.

Weeks before the attempt on Trump’s life, the Secret Service received intelligence warning of an Iranian plot to kill the former president, according to federal law enforcement officials. But while experts are skeptical of any connection between the 20-year-old would-be assassin Thomas Matthew Crooks and Iran, they haven't been able to access some of his communications 18 days later.

So it remains unclear who he was talking to, if anyone, and what topics would have been discussed.

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"I think we've experienced a range of returns because some of the applications that he was using online were encrypted in nature," FBI Deputy Director Paul Abbate said during a Senate hearing Tuesday under questioning from South Carolina Republican Lindsey Graham.

The FBI has gained access to some of his emails, he said, but some of his encrypted communications remained undecipherable Tuesday.

"If he were talking to some foreign power, and I don't think any foreign power would hire this guy, by the way, so I'm not overly worried, but if this was some great plot by the Iranians, how do we solve this?" Graham asked.

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"Senator, you know, as we've been saying, we need a solution that provides lawful access," Abbate said.

The FBI and other law enforcement agencies have been seeking – unsuccessfully – backdoor access to encrypted data for years. And while lawmakers have been reluctant to demand it facing privacy and due process concerns, apps that facilitate encrypted communications have become popular not only with private individuals, but also terrorist groups, criminal cartels, drug traffickers and other criminals. They are easy to obtain on Apple and Android phones – and often free to use.

Thomas Matthew Crooks, 20, climbed on top of a roof less than 150 yards from Trump during a rally in Butler, Pennsylvania on July 13. Armed with an AR-15 rifle, he tried to kill the former president, barely missing his head but grazing his ear, killing one rally attendee and critically wounding two others. The shocking security failure spawned a number of investigations into both lapses in event security and the gunman's motivations. The attack came amid a heightened threat from Iran.

"The terrorist regime of Iran have been targeting people, our country, for many, many years now," Abbate told Sen. Richard Blumenthal, D-Connecticut, during the hearing. "We've talked about that here before. We know, publicly, they've targeted former President Trump;. They've called for his assassination."

However, when it comes to the July 13 attack, he said investigators have found no information that Crooks had any co-conspirators, "foreign or domestic."

"But I wanna be clear: We have no evidence of that. We're not ruling anything out. We're looking into all possibilities and leaving our minds open to that," Abbate told lawmakers Tuesday.

What if Crooks did have ties to a terror group, or violent cartels? The high-ranked FBI official warned Senate committee members Tuesday that the bureau needs a way to gain "lawful access" to a suspect’s encrypted communications.

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Many current and former law enforcement officials have argued that a hard-coded decryption method would aid them as they try to protect the public from major threats. On the other hand, opponents of a digital master key say it would create both a privacy risk for law-abiding individuals and a vulnerability that criminal hackers could exploit.

The government has unsuccessfully been seeking backdoor access into Big Tech encryption for years, according to Paul Mauro, a retired NYPD inspector and counterterrorism expert.

"Encrypted or not, you have to do what we’ve been doing in our system for 250 years — get a warrant," Mauro told Fox News Digital. "But by creating encryption that the creators themselves cannot decrypt — you have obliterated the role of the judges. Really, the only ‘judge’ now is whoever can decrypt."

But it’s also a "slippery slope," said David Gelman, a former prosecutor turned defense attorney in the Philadelphia area.

"That would erode all confidence in companies like Apple and Signal, etc.," he told Fox News Digital.

Both companies tout their protection of users' privacy as a selling point.

"There is only so much that should be able to be accessed," Gelman said. "[It’s] too much of an invasion of privacy."

Even if a court were to issue a warrant, however, with unbreakable encryption, it doesn’t matter if government specialists can’t crack the code.

FBI officials said earlier this week that Crooks’ encryption has been a challenge, including foreign-based encrypted email accounts. The FBI said there has been no indication of foreign involvement with Crooks' plot, but investigators have left the question open until they get a look at his hidden communications.

Investigators are also continuing to examine information that they have received, including activity from social media accounts that may have been used by the gunman and espoused what Abbate called "differing points of view."

On Friday, Trump revealed he would be returning to Butler for another rally to honor the bystander who Crooks killed with a stray bullet.

Crooks fatally struck 50-year-old Corey Comperatore and critically wounded David Dutch, 57, and James Copenhaver, 74.

Fox News’ Sarah Rumpf-Whitten contributed to this report.