Congressman Joaquin Castro was frustrated and angry. The Texas Democrat had just learned that the Republicans were pulling the plug on the tortured, bastardized House Intelligence Committee investigation into Russian interference with the 2016 presidential election. “I just saw—on the news—that Mike Conaway is announcing that they’re wrapping up the investigation and the committee is just going to do a report,” Castro, a member of the committee, says. “There are unanswered questions on collusion, money laundering, and obstruction. Not following any of these leads is an abject failure for the American people. So thank God that Bob Mueller and the Senate are still conducting their investigations.”

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The Senate probes have plenty of their own partisan constraints. Mueller, however, has been barreling ahead. In mid-February the special counsel unveiled the indictment of 13 Russians and 3 Russian-related companies, charging them with waging a sprawling effort to plant thousands of bogus Internet items and stage sham rallies in support of the Trump campaign. Last month’s indictment, though, did more than identify alleged foreign criminals: it laid a foundation for Mueller’s next big move. He’s likely to target another batch of Russians, this time for hacking into the Democratic National Committee, Republican National Committee, and Clinton campaign computers—and outside investigators expect Mueller will also name Americans who may have helped the Russians distribute the hacked materials.

Castro has a unique window onto the special counsel’s possible course and sees careful calculation in its unfolding. “By indicting the 13 Russians first,” he says, “Mueller laid the groundwork to show that there is this malignant force out there that was interfering with the American elections. Once everybody can appreciate that, then he moves forward and says, ‘O.K., these are the Americans that were helping these bad people.’”

Possible big-name targets—all of whom have consistently denied conspiring with or aiding the Russian election-meddling—include Roger Stone Jr., the longtime Trump adviser; Guccifer 2.0 correspondent, and gleeful dirty trickster; Cambridge Analytica, the data-mining company; and Brad Parscale, who ran the Trump campaign’s social-media operation. But Mueller has so far proven adept at finding previously arcane operatives and forcing them to plead guilty—including foreign-policy adviser George Papadopoulos, Dutch attorney Alex van der Zwaan, and Rick Gates, the right-hand man to Paul Manafort, Trump’s former campaign chairman who has financial ties to Eastern Europe.

“The big question here is whether the Russians had any help in distributing the hacked material,” Castro says. “And it’s really any guidance or direction or information sharing or data sharing with any Americans. These are very sophisticated folks, so it doesn’t take a lot of interaction necessarily to get them springing into action. They don’t need 10,000 e-mails going back and forth. You just need to point them in the right direction. Mueller is very good at uncovering people who had a role who are not obvious—middlemen or obscure operators. So I assume he’s got those to come. And getting those guys to turn on their superiors, that could be key. If you’re looking just for a paper trail with the main cast of characters, you may not find it. But you’ve got all these underlings who may have been given a direction, ‘Hey, go figure this out.’ Parscale had 100 people under him here in San Antonio for the digital operation.” Including helpers from Twitter, Facebook, and Google. “We’ve heard that the Trump campaign went through dozens of Facebook embeds until they settled on people who they found ‘loyal,’” one House investigator says. “Has Mueller talked to them, or to the Google and Twitter embeds? Seems logical.” (The special counsel’s office and Twitter declined to comment; Facebook and Google did not return requests for comment.)

Whoever Mueller names in the next indictment will also be contending with a new, sinister extra-legal worry. The poisoning of former Russian spy Sergei Skripal and his 33-year-old daughter in England has sent chills through the ranks of witnesses and possible cooperators—just as it was intended to do. “When countries do a spy swap, you’re basically agreeing that we’re going to let you take this person, and they’re allowed to live in your country,” says Michael Carpenter, a former deputy assistant defense secretary with an expertise in Russian affairs. “It’s not, ‘We’re going to let you take this person but we’re going to continue to try and kill them.’ That’s an unwritten rule of spycraft.”

When it comes to the 2016 American presidential election, however, the Putin regime looks increasingly unwilling to follow rules of any kind. “I suspect Skripal was talking to Christopher Steele or someone on Christopher Steele’s team,” Carpenter says, referring to the former British spy who in 2016 compiled the “dossier” of alleged ties between Trump and Russia. “That is why this got Putin’s ire up, and so that’s why they went after him. The Kremlin’s No. 1 goal here is to intimidate anyone that may have talked to Steele or any of his associates, or who might cooperate with Mueller. Yeah, it’s pretty ominous. It’s a really, really bad development.”

This post has been updated to restore part of a quotation from Joaquin Castro.