Mark Epstein attends the Sing for Hope Gala in New York on October 24, 2016. Photo: Owen Hoffmann/Paul Bruinooge/Patrick McMullan

Jeffrey Epstein’s apparent suicide over the weekend likely means that his criminal case will not proceed. But in a statement Saturday, Attorney General William Barr issued a sharp warning: “Any co-conspirators should not rest easy.” There has been plenty of speculation as to whom may be held responsible for helping or enabling Epstein, from his alleged madam, Ghislaine Maxwell, to the attorneys who handled his business dealings for decades. While he has not been accused of involvement in the sex offender’s crimes, new details about Jeffrey’s younger brother, Mark Epstein, have raised questions about their business ties, and how close the pair really were.

Jeffrey and Mark grew up together in the Sea Gate community in Brooklyn. Mark, who is a year-and-a-half younger than his brother, went on to start a silk-screening business and, having “semi-retired” at age 39, he dabbled in real estate and philanthropy. In 2009, Mark was named chair of the Cooper Union board. That ended with his resignation in 2015 after the school’s controversial decision to begin charging students tuition for the first time in the college’s 150-year history.

The Epstein brothers’ relationship in adulthood is opaque. In filings, the U.S. Attorney for the Southern District had said Jeffrey had “no known immediate family” and “no meaningful family ties.” Yet in a 2009 deposition, Mark said that he knew Donald Trump flew on Jeffrey’s plane “numerous times” and that he’d personally seen Trump on his brother’s plane once. “They were good friends,” Mark recently told the Washington Post. “I know [Trump] is trying to distance himself, but they were.” After Jeffrey pleaded not guilty last month to sex trafficking of minors, Mark offered his Florida home to guarantee his brother’s bond.

Last month, Mark told Crain’s New York that he had no business connection to his brother, but documents show that Mark’s real-estate business, Ossa Properties, is linked to J. Epstein & Co., the company through which Jeffrey managed retail billionaire Leslie Wexner’s assets. An employee at Ossa Properties, Jonathan Barrett, was also an asset manager for J. Epstein & Co. Mark told Crain’s that the connection between the two companies was “a mistake,” reminiscent of the “recording error” that linked private-equity tycoon Leon Black’s foundation to Jeffrey years after he was convicted of soliciting a minor for prostitution in Florida.

At the center of the curious connection between Mark’s real-estate company and his brother is a condominium at 301 East 66th Street, a 15-minute walk from Jeffrey’s mansion on East 70th Street. Mark owns the majority of the units in the bland, 16-story, 200-unit building, which he purchased from Wexner in the early 1990s. Mark has said his brother does not own a share of the building, but for years Jeffrey allegedly housed friends, employees, and associates in apartments in the building, including models connected to MC2, the modeling agency in which he invested. (The Daily Beast recently reported that Mark also began dabbling in the modeling industry around the same time Jeffrey invested in MC2.) In 2010, a former MC2 employee said in a sworn statement that the MC2 models were in fact victims of Epstein’s underage sex-trafficking ring.

“Jeffrey rents several apartments there where he keeps his girls, alleged models for the MC2 agency he owns,” Florida attorney Brad Edwards, who represents a number of Epstein’s alleged victims, told “Page Six” in 2009. “But Mark acts like he doesn’t even know his brother.”

In addition to the models, the Jeffrey Epstein associates who also rented apartments or ran businesses out of the building included an ex-girlfriend, his pilot, and his MC2 business partner, Jean-Luc Brunel. (One of Epstein’s alleged victims has claimed she was forced to have sex with Brunel.) Two other potential Jeffrey Epstein co-conspirators, Nadia Marcinkova and Sarah Kellen, both operated businesses that were registered at the building. Former Israeli prime minister Ehud Barak, who was photographed outside Jeffrey’s townhouse in 2016, also regularly spent time at the condo.

“I don’t live in that building,” Mark told Crain’s. “I don’t monitor who uses those apartments.”