The last major Russian spy arrested on U.S. soil was busted for seeking the kind of information retired Lt. Gen. Michael Flynn has been accused of dishing out.

During a White House press conference on Thursday, President Donald Trump defended Flynn, his former national security adviser, for talking about U.S. sanctions against Moscow with Russian Ambassador Sergey Kislyak while Barack Obama was still in office. It’s an act that may have put Flynn in legal jeopardy; The Washington Post reported Thursday that Flynn denied to the FBI having such conversations, despite evidence that he did.

Recently filed court documents show just how important information about sanctions was to Russian intelligence.

Those documents involve a two-year-old case against Evgeny Buryakov, a Russian bank employee who admitted to being an unregistered agent of Russian intelligence in the U.S. Buryakov pleaded out and the case never went to trial. But case filings show that the SVR, Russia’s foreign intelligence service, was keenly interested in the U.S. government’s attempts to use financial sanctions to retaliate against Russian military aggression.

His handlers asked Buryakov to look for information on the “effects of economic sanctions on our country,” according to court documents, and he complied. The FBI sent an undercover operative to keep him interested.

In August 2014, an undercover agent showed Buryakov a document from the Treasury Department marked “Internal Treasury Use Only,” that “contained information regarding Russian individuals subject to sanctions,” according to court filings. (It’s not clear whether the papers in question were actual internal Treasury Department memos.) Buryakov told the undercover that he wanted more information.

A few weeks later, the undercover agent and a confidential source fed him another document, telling him that “the Treasury Department was using the document in connection with its deliberations regarding additional sanctions,” which Buryakov promptly fed to his handlers at Russia’s foreign intelligence service.

That is exactly the kind of information that would be useful to foreign spies, said Zachary Goldman, a former Treasury and Department of Defense official who’s now the executive director of the Center on Law and Security at New York University.

The U.S. authorized sanctions against Russia relating to its annexation of Crimea in March of 2014 and began a crackdown against individuals and a Russian bank. In the period Buryakov was fishing, then, his overseers would have wanted to know which entities or people would be sanctioned next.

“In that period, the first half of 2014, the Russian government was very interested in figuring out what we were going to do,” Goldman said.

When Flynn spoke to Kislyak, the Russian ambassador, in December 2016, the Russians would’ve been in much the same situation.

The sanctions announced by the Obama administration that month exercised a relatively new authority enacted by the president in April 2015. Obama’s order on cyberattacks was originally in response to Chinese attacks on the private sector, and later broadened to be applicable to the Russian attempts to interfere in U.S. elections.

Finding out who was going to be targeted, and what the policy would be like under the next administration, would have been a top priority for all actors of Russian intelligence. They come in various categories: Some, like Buryakov, conduct espionage in secret while pretending to be an ordinary employee of a foreign company, while others construct alternate identities and lay in wait for years. The third category come here under diplomatic cover, having, in effect, a dual role as diplomats and spies.

“It seems that the reports are that there was some kind of suggestion that Flynn gave Kislyak, along the lines of, don’t worry about these sanctions, when we take office, things will improve significantly,” Goldman said. “And undoubtedly, that’s something they would want know.”

The point of sanctions is to change another country’s behavior, Goldman added.

“If you were the Russians, you would want to know what the trigger for new sanctions would be, and what the catalyst for the removal of sanctions would be,” he said. “Whether that’s what Flynn discussed with Kislyak, I have no idea.”

Details about the conversations, and whom Flynn misled about their content, are still emerging. But we know that when the Obama administration exiled 35 diplomats and shut down a Russian compound on Long Island, Russian officials announced they would not be following suit.

At a press conference on Thursday, however, Trump backed Flynn’s right to discuss that matter.

“Very simple. Mike [Flynn] was doing his job,” Trump said. “He was calling countries and his counterparts. So, it certainly would have been OK with me if he did it.

“I would have directed him to do it if I thought he wasn’t doing it,” Trump added.