The missive is the latest salvo from the U.S. government and its allies in a battle over encryption that has long pit Silicon Valley against Washington when it comes to striking the right balance between consumer privacy and national security concerns.

As tech companies such as Facebook, Apple and Google have rolled out increasingly strong privacy protections, which would limit access to data even when agencies have a warrant, law enforcement and national security officials have complained they are increasingly unable to investigate crimes and terrorism cases. Encryption became a national political issue following the 2015 San Bernardino, Calif., terrorist attack when investigators could not unlock the shooter’s iPhone.

“Security enhancements to the virtual world should not make us more vulnerable in the physical world,” the four officials wrote in their letter, which precedes a Justice Department event on Friday that will focus on how online encryption affects investigations into child exploitation cases.

“We must find a way to balance the need to secure data with public safety and the need for law enforcement to access the information they need to safeguard the public, investigate crimes, and prevent future criminal activity,” they said.

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The letter is specifically responding to Facebook’s plan for a more privacy-focused social network that Zuckerberg laid out in a March 6 post when he announced deploying more end-to-end encryption.

In that post, he said, “I believe working towards implementing end-to-end encryption for all private communications is the right thing to do. Messages and calls are some of the most sensitive private conversations people have, and in a world of increasing cyber security threats and heavy-handed government intervention in many countries, people want us to take the extra step to secure their most private data.”

Yet officials say that approach will even harm work Facebook itself has done to report criminal activity on the platform. In their letter, the officials note Facebook made 16.8 million reports to the U.S. National Center for Missing & Exploited Children concerning images of child abuse or suspicious attempts by users “to meet children online and groom or entice them into sharing indecent imagery or meeting in real life.” If it deploys encryption throughout its services, they say, this kind of monitoring will no longer be possible.

In response to the letter, Facebook said it is working with child advocacy groups, as well as tech experts and government officials, before adding increased security and privacy measures to its apps.

“End-to-end encryption already protects the messages of over a billion people every day,” a Facebook spokesperson told POLITICO in a statement. “It is increasingly used across the communications industry and in many other important sectors of the economy. We strongly oppose government attempts to build backdoors because they would undermine the privacy and security of people everywhere.”

Privacy advocates and technologists have long argued that tech companies and social media platforms can’t create special access for law enforcement while still protecting consumers’ security online.

"If we could design a technology to tell good guys from bad guys, we could exclude bad guys from these systems,” said Joseph Lorenzo Hall, chief technologist at the Center for Democracy and Technology. “But we can't. We don't try to design sidewalks that crumble underneath the feet of bad guys."

What’s more, he said, the government’s approach of using violence against children as the reason to argue against encryption amounts to fear-mongering. "Frankly I think they've seen child sexual exploitation and sex trafficking as a wedge issue,” he said.

Despite officials’ complaints about more widespread encryption on Facebook’s messaging apps, people are increasingly gravitating toward services that offer encryption.

"The trend in private messaging is towards end-to-end encryption,” said Thomas Rid, a cybersecurity expert and professor at Johns Hopkins University’s School of Advanced International Studies. “By and large, that is a positive and good trend."

But he recognizes this trend means that law enforcement may be blocked from getting access they need for investigations. There’s a downside "because you lose visibility into potentially criminal behavior, and this is an example of it. It also means that people who are in more vulnerable situations are protected from their governments. You have to balance out the costs."

Along with the letter to Facebook, officials also plan to announce an agreement that will allow British law enforcement to demand major American tech companies hand over electronic data relating to terrorism, child exploitation and other serious criminal offenses.

The new agreement “will enhance the ability of the United States and the United Kingdom to fight serious crime — including terrorism, transnational organized crime, and child exploitation — by allowing more efficient and effective access to data needed for quick-moving investigations,” Barr said in a statement reviewed by POLITICO also set to be released on Thursday evening in the U.S.

“Only by addressing the problem of timely access to electronic evidence of crime committed in one country that is stored in another, can we hope to keep pace with twenty-first century threats,” he added.

In a statement obtained by POLITICO, Patel said the data agreement “will dramatically speed up investigations, allowing our law enforcement agencies to protect the public.”

Prior to approving the new agreement, law enforcement data request for communications data had to be submitted and approved by central governments via mutual legal assistance — a process that could take anywhere from six months to two years. The agreement will shrink that amount of time to weeks, possibly days.

Steven Overly contributed to this report.