Brad Heath

USA TODAY

WASHINGTON — The FBI guards its high-tech secrets so carefully that officials once warned agents not to share details even with federal prosecutors for fear they might eventually go on to work as defense attorneys, newly disclosed records show.

A supervisor also cautioned the bureau’s “technically trained agents” in a 2003 memo not to reveal techniques for secretly entering and bugging a suspect’s home to other agents who might be forced to reveal them in court. “We need to protect how our equipment is concealed,” the unnamed supervisor wrote.

The records, released this year as part of a Freedom of Information Act lawsuit, offer a rare view of the extent to which the FBI has sought to keep its most sensitive surveillance capabilities secret, even from others within federal law enforcement. That secrecy remains a common feature of the FBI’s most sophisticated investigations, including recent cases in which it cracked the encrypted iPhone of one of the gunmen in last year’s San Bernardino terror attacks and breached the anonymous Tor computer network.

But it has also alarmed some lawyers and privacy advocates, who worry that the secrecy makes it difficult for courts to scrutinize the agency’s tactics.

“For the courts to do their jobs they need to know what they’re being asked to approve,” said Chris Soghoian, the principal technologist for the American Civil Liberties Union. “What we’ve seen time and time again is that when courts do finally learn what they’ve been authorizing, they’re often not very happy about it.”

The newly-released memos circulated in the FBI’s Minneapolis field office in 2003. They came to light this year in the midst of a Freedom of Information Act lawsuit by Ryan Shapiro, a doctoral candidate at the Massachusetts Institute of Technology who is seeking information about how the government investigated animal rights activists. The FBI did not release the names of the agents who sent the messages.

In one, an official warned agents in Minneapolis that the FBI’s Engineering Research Facility — where some of the bureau’s most sophisticated tech secrets are hatched — had “expressed concerns about Tech Agents revealing technical details to Case Agents and especially to AUSAs,” using the acronym for assistant U.S. attorneys. “There have been several instances of AUSAs becoming familiar with our technology, then resigning and becoming defense lawyers.”

As a result, the official warned, “the tech guys may be a little mum about how we’re doing things," adding officials would "try to get it cleared" if prosecutors wanted a fuller technical briefing.

That approach was “fairly consistent with FBI policy,” said Michael Tabman, the former head of the FBI’s Minneapolis field office. “The point is that there’s usually no need for the case agents or the prosecutors to know how something was done.”

Still, some former prosecutors said they found it troubling that agents would be unwilling to share technical details — particularly in eavesdropping cases that require extensive consultations with Justice Department attorneys. “The fact that somebody may at some point go over to the other side is not a legitimate basis to withhold relevant information,” said Rachel Paulose, who was Minnesota’s U.S. attorney from 2006 to 2008.

A spokesman for the FBI, Chris Allen, said he could not comment on memos from 2003, but added “certainly it is true that details involving sensitive techniques are not widely shared.”

When local police forces began deploying sophisticated cellphone tracking devices, for example, the FBI insisted that they first sign non-disclosure agreements in which they promised not to reveal the technology publicly.

That promise drew a skeptical reaction from courts. Last month, Maryland’s second-highest court ruled that police needed to obtain a warrant before using the devices, seeming dismayed that state officials still were unwilling to say how they worked. If judges are to rule on whether a police search is lawful, the court said, “it is self-evident that the court must understand why and how the search is to be conducted.”

A similar issue surfaced this year after the FBI acknowledged it had pierced the anonymous Tor computer network. Agents did that by taking over a large child pornography site that operated on the so-called “dark web” and infected it with malware to identify people who were sending and receiving illicit images. Special Agent Daniel Alfin, described the malware in a court filing as “lines of code that are able to take advantage of a software vulnerability,” but so far the government has refused to say more.

A federal judge in Tacoma, Wash., ordered the Justice Department to reveal to defense lawyers in one case how that malware worked so that they could challenge the evidence the FBI obtained as a result. The department balked at that order, saying in a March court filing that it was “taking the unusual step of asking this Court to reconsider,” for reasons that it asked the court to keep secret.

A hearing on that request is scheduled for May.