Keith Gartenlaub peers through blinds in the living room of his home in Southern California on March 23. An espionage investigation led to Gartenlaub being convicted of the possession of child pornography, which he denies. (Stuart Palley/For The Washington Post)

FBI agents entered Keith Gartenlaub’s home in Southern California while he and his wife were visiting her relatives in Shanghai. Agents wearing gloves went through boxes, snapped pictures of documents and made copies of three computer hard drives before leaving as quietly as they had entered.

The bureau suspected that Gartenlaub was a spy for China.

The FBI had obtained a secret search warrant to enter the house, citing national security grounds. The agents were searching for evidence that Gartenlaub, an information technology manager at Boeing, had leaked computer information about the defense contractor’s C-17 military transport plane to people acting on behalf of China.

But since the search in January 2014, no spy or hacking charges have been brought against him.

Instead, seven months later, he was charged with the possession and receipt of child pornography. He has denied the charges, but a jury convicted him in December.

Keith Gartenlaub points to a photo of himself and his wife at their home in Southern California. (Stuart Palley/For The Washington Post)

Gartenlaub’s case highlights how exceptional powers given to the government in recent years to gather information about suspected terrorist or espionage threats without some of the traditional safeguards for a defendant’s rights are now leading to charges in more routine criminal ­cases.

Over the past 15 years or so, the wall between U.S. intelligence officials and criminal prosecutors has fallen, making it easier for them to share information, especially to fight terrorism. And under the Foreign Intelligence Surveillance Act (FISA), defendants are generally unable to effectively challenge the warrants that authorized the search or surveillance because they are not permitted to see them or the underlying application on national security grounds.

Gartenlaub’s attorneys are troubled that what began as an espionage case — one that resulted two weeks ago in a guilty plea by a Chinese businessman with no connection to their client — morphed into a child pornography prosecution.

In Gartenlaub’s case, the government made sealed filings, so neither the defense nor the public was able to see them. Based on the secret filings, the judge held that the government had shown probable cause that the house to be searched belonged to “an agent of a foreign power” or a spy.

“There has, over the last decade-plus, been an erosion of the formerly bright line between foreign intelligence surveillance and investigation for criminal prosecution,” said Jennifer Daskal, a former official in the Justice Department’s national security division who teaches law at American University.

In criminal ­cases, by contrast, a defendant and his attorneys are generally entitled to see an affidavit for a warrant and challenge the grounds for its issuance before a judge. Gartenlaub wants to see the warrant in his case so he can challenge it as based on false information and therefore invalid.

Keith Gartenlaub adjusts blinds to maintain privacy inside his home. (Stuart Palley/For The Washington Post)

“The government is increasingly using national security tools to investigate domestic criminal ­cases, bypassing key constitutional protections,” said Patrick Toomey, a staff lawyer with the American Civil Liberties Union. “This problem is only compounded in the digital age, where the FBI is collecting vast amounts of our data for intelligence pur­poses but then goes sifting through all that information in unrelated criminal investigations.”

In a case in Philadelphia last year, for instance, the government used a FISA order to obtain evidence on a Temple University professor who they apparently suspected was sharing technology with China, but they indicted him on garden-variety wire fraud charges­ before eventually dropping the case. In an Iowa case, the government used a FISA order to gather information about a Chinese businessman suspected of stealing patented corn seeds from farm fields. In 2013, he was indicted on charges of theft of trade secrets. He pleaded guilty this year to one count of conspiracy to steal trade secrets.

Federal prosecutors in Gartenlaub’s case insist that they followed the law.

“The issue of the FISA warrant was the subject of an extensive pre­trial briefing and an order from the judge finding that the orders were lawfully issued and did not violate the defendant’s due process rights,” said Thom Mrozek, a spokesman for the U.S. attorney’s office in Los Angeles.

The judge specifically found that the pornography material “obtained pursuant to FISA was lawfully acquired” and did not violate the defendant’s Fourth Amendment rights, he said. The court also found that “there is no indication of any false statements having been included in the FISA materials.”

Mrozek said the fact that the child pornography case began as a national security investigation does not lessen its severity.

“When law enforcement lawfully obtains evidence of a serious crime, in this case a crime against children, we will pursue further investigation of that crime,” he said.

Justice Department officials added that Congress has always intended that information obtained through intelligence authorities could be used in criminal prosecutions. “It would be irresponsible for the government to ignore evidence of criminal wrongdoing when such evidence is lawfully collected,” said Justice Department spokesman Marc Raimondi.

In Gartenlaub’s case, the defense unsuccessfully argued that he could not be linked to identical copies of child pornography videos found on four hard drives in his house. Two of the hard drives had been in a computer that was kept at a beach house where numerous people had access to it, Gartenlaub said.

“They claim I’m a spy and a pervert, and I’m neither one,” Gartenlaub said in an interview from his home in Riverside County, a house he may not leave without permission while awaiting sentencing.

Jeff Fischbach, a forensic technologist for the defense, said there is no evidence that the child pornography was ever seen by anyone who used the computer, much less Gartenlaub.

The government’s own forensic expert, Bruce W. Pixley, said he could not find any evidence of the material being downloaded onto any of the computers, the defense noted. That means it had to have been copied onto the computer — but by whom is unknown.

Prosecutors are seeking a 10-year prison term for Gartenlaub, who has asked Judge Christina A. Snyder of the U.S. District Court for the Central District of California to overturn the verdict or order a new trial. A hearing is scheduled for this month.

Gartenlaub, 47, was fired in August 2014 and has been unemployed since. His attorney said his defense was hampered by an inability to obtain basic information about how the evidence was obtained and on what specific grounds the warrant was issued.

“We’ve always cherished the right to confront and cross-examine our accusers and examine the evidence that’s used as the basis for a search of our homes,” said Mark J. Werksman, Gartenlaub’s attorney. “And to be told, ‘We went in. We had good reasons. We’re not going to tell you why. Trust us,’ is alarming. Especially when the case becomes a run-of-the-mill criminal case.”

In February 2013, the FBI emailed Gartenlaub that it was investigating a data breach and wanted to talk to him. Over two days at Boeing’s facility in Long Beach, Calif., and a third day at its Huntington Beach facility, agents interviewed Gartenlaub about the C-17 program; his team of engineers was also questioned. Two agents showed him a copy of an intercepted email. The communication described information on the C-17 that was apparently being sought by the Chinese.

Gartenlaub told them that he had no idea who wrote it or why.

The next time he heard about the C-17 was more than a year later, in June 2014, when he saw the news about the arrest of the Chinese businessman, Su Bin. When he reviewed Su’s arrest warrant, he realized that the email excerpt he had been shown 16 months earlier had been sent by Su.

Still, Gartenlaub had no sense that he was a target. “Why would I think I would be under suspicion?” he said.

The only thing he could think was that his wife, who was born in China and became a U.S. citizen and was a member of an Orange County Chinese business association, somehow made the FBI suspicious.

In an affidavit for a warrant for the couple’s emails, separate from the national security warrant, agent Wesley Harris stated that Gartenlaub was the “nationwide Unix military administrator for Boeing,” suggesting that that position would allow him to log into C-17 data, Gartenlaub said.

According to two Boeing colleagues, who spoke on the condition of anonymity because they were not authorized to talk to the media, there is no such job at the company. And Gartenlaub was, in any case, an IT manager. Moreover, they said, the breached files were accessible through servers in the field, such as at Air Force ­bases. These were not servers that Gartenlaub or his team of engineers who supported the plane’s designers had access to, they said.

Then on a Monday in late August, two FBI agents came to speak to Gartenlaub at the Huntington Beach facility. They showed him pictures of himself and his wife with some of her acquaintances whom Gartenlaub couldn’t remember.

Two days later, the agents returned. This time they handcuffed him.

During his initial appearance in a federal courthouse in Santa Ana, Calif., the prosecutors indicated a willingness to reduce or drop the child pornography charges if he would tell them about the C-17, said Sara Naheedy, Gartenlaub’s attorney at the time.

“They said what they really wanted was information about the C-17 Chinese hacking situation,” Naheedy recalled.

All along, Werksman said, the government suspected Gartenlaub was working with Su. “They triangulated Keith as the guy at Boeing who would have been Su Bin’s inside source,” he said.

That suspicion is “ridiculous,” Gartenlaub said.

“I’ve been a good Boeing employee for years. Just because I married somebody from China doesn’t mean I’m going to betray my country,” he said. “If they think I’m a spy, then charge me with it.”

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