The F.B.I.’s search of Michael Cohen’s Rockefeller Center office is the sexier raid—literally, given the involvement of a porn star. But the bureau’s visit to a nondescript storage locker in suburban Virginia may turn out to be more significant to Robert Mueller’s investigation into possible collusion between Donald Trump and Russia. Certainly the renter of locker 3013, former Trump campaign manager Paul Manafort, is already finding those seized documents troublesome.

Mueller’s investigators have confiscated thousands of pages of evidence related to Manafort, and used them to indict the longtime Republican operative and his former deputy, Rick Gates, on 44 counts spread across two different jurisdictions. (On Wednesday, Mueller asked the presiding judge to issue 35 sets of subpoenas to witnesses for the first trial.) But it is the contents of the Alexandria storage locker that Manafort’s lawyers have chosen to try to suppress first, filing a motion late last week in federal district court. Perhaps, as the motion argues, the F.B.I. illegally gained access to the Public Storage facility in order to gather information for a search warrant—a tactic known in law enforcement as “sneak and peek.” And it’s standard, responsible practice for white-collar defense lawyers to push to exclude as much evidence from a trial as possible: on Monday, they filed a new motion, this one to suppress records removed from Manafort’s condo.

There’s also the chance their client was hiding something damaging in those boxes and binders. “Sure, it behooves defense lawyers to very carefully evaluate everything that happened around a search warrant,” says Joyce Vance, a former U.S. attorney in Alabama. “But the main reason defense lawyers challenge the execution of the procedures is that search warrants tend to come up with the good stuff.”

Manafort’s murky transactions range from New York City real-estate deals to shell companies that stashed money in obscure Caribbean-island banks. Mueller is especially interested, however, in piecing together the paper trail associated with Manafort’s multi-million-dollar work for right-wing Ukrainian political figures, particularly its pro-Russian former president, Viktor Yanukovych. If the documents in the Virginia storage locker shed any light on Manafort’s Eastern European maneuvers, his lawyers have good reason to be concerned. They have plenty of other headaches, even if those papers turn out to be Manafort’s grocery lists, however. “The pending money laundering and related cases are very strong,” says Randall Eliason, a former federal prosecutor who now teaches about white-collar crime at George Washington University Law School. “These types of cases are really hard to defend against, because they don’t rely on witness credibility or things that are easily attacked. You either declared the bank account or you didn’t. You registered as a lobbyist or you didn’t.”

Manafort’s game plan to refute Mueller is mysterious. (His spokesman, citing a judge’s gag order, declined to comment.) Yet after a year of being under increasing pressure from the special counsel—including seeing Gates plead guilty and begin cooperating with investigators—he has shown no sign of wanting to cut a deal with Mueller, and says he is determined to go to trial, in Virginia this July and in Washington this September. Manafort might be fighting back for the simplest reason: that he is innocent, as he has long and consistently maintained. But the skirmish over the storage locker could offer a clue to his larger counter-offensive, one that is more political than legal. Even if his lawyers fail to knock out any evidence, they are laying the groundwork for putting the F.B.I. and Mueller on trial, accusing the investigators of being hopelessly biased and reckless.