As federal investigators pore over the Trump campaign’s ties to Russia, the work of ex-British spook Christopher Steele continues to loom large. At least part of Robert Mueller’s probe has been informed by Steele’s infamous dossier, which alleges substantive ties between Donald Trump and the Russian government. Though Trumpworld has cast doubt on the document’s legitimacy, with former Trump aide Carter Page dubbing it the “dodgy dossier,” Steele has stood by his research and, reportedly, has offered up another tip: zero in on the president’s foreign real-estate deals.

In December of last year, Steele informed Luke Harding, a journalist for the Guardian, that “the contracts for the hotel deals and land deals” between Trump and individuals with the Kremlin ties warrant investigation. “Check their values against the money Trump secured via loans,” the former spy said, according to a conversation detailed in Harding’s new book, Collusion: Secret Meetings, Dirty Money, and How Russia Helped Donald Trump Win. “The difference is what’s important.”

According to his book, Steele did not elaborate on this point to Harding, but his implication was clear: it’s possible that Trump was indebted to Russian interests when he descended Trump Tower’s golden escalator to declare his candidacy. After the real-estate mogul suffered a series of bankruptcies related to the 2008 financial crisis, traditional banks became reluctant to loan him money—a reality he has acknowledged in past interviews. As a result, the Trump Organization reportedly became increasingly reliant on foreign investors, notably Russian ones. As Donald Trump Jr. famously said in 2008, “Russians make up a pretty disproportionate cross section of a lot of our assets. We see a lot of money pouring in from Russia.”

The significance of such transactions is not lost on Special Counsel Robert Mueller. Citing a person familiar with the F.B.I. probe, Bloomberg reported in July that Mueller’s team is investigating a series of deals Trump struck, including the Trump Organization’s failed SoHo development that involved Russian nationals, the 2013 Miss Universe pageant in Moscow, and the president’s sale of a Palm Beach estate in 2008. All three deals have drawn scrutiny for their ties to Russian interests; as Craig Unger outlined for the Hive, the 2014 Trump SoHo development is likely of interest to Mueller thanks to the involvement of Felix Sater—a Moscow-born, Russian-American businessman who did time for stabbing a man in the face with a margarita glass—and the now-defunct company he worked for, the Bayrock Group. Similarly, Russian developer Aras Agalarov, whose son Emin helped broker the controversial Trump Tower meeting last June between Donald Trump Jr. and Russian lawyer Natalia Veselnitskaya, paid $20 million to bring Miss Universe to Moscow. And Russian fertilizer magnate Dmitry Rybolovlev bought the Florida mansion for a staggering sum of $95 million in 2008—despite Trump having paid just $41 million for the property four years prior.

Trump has cautioned that he would view any attempt by Mueller to dig into his past business deals as out of bounds. But the former F.B.I. director has a broad mandate from the D.O.J. to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign” and “any matters that arose or may arise directly from the investigation”—suggesting that Trump’s deals with Russians fall under Mueller’s purview. Nor is Mueller’s tack in following the money limited to Trump. The indictments the special prosecutor brought against former Trump campaign manager Paul Manafort and his deputy and longtime business associate Rick Gates included conspiracy to launder money and seven counts of improper foreign banking and financial reporting. (Both Manafort and Gate have denied the charges.)

Since media outlets published the Steele dossier last January, Republican lawmakers and conservative pundits alike have sought to discredit it. In recent weeks, Trumpworld has latched onto the revelation that the Democratic National Committee and the Clinton campaign indirectly bankrolled Steele’s investigative work—which he conducted for Washington-based intelligence firm Fusion G.P.S.—through the law firm Perkins Coie. They have argued that the dossier’s origins not only make it invalid, but are indicative of a larger anti-Trump conspiracy. Steele, however, stands by his work. While the former MI6 agent acknowledged that no piece of intelligence is 100 percent airtight, Harding noted that Steele told friends he believes the 16 memos he delivered to Fusion to be “70 to 90 percent accurate.”