It was the summer of 2012, and Jared Kushner was headed downtown.

His family’s real estate firm, the Kushner Companies, would spend about $190 million over the next few months on dozens of apartment buildings in tony Lower Manhattan neighborhoods including the East Village, the West Village and SoHo.

For much of the roughly $50 million in down payments, Mr. Kushner turned to an undisclosed overseas partner. Public records and shell companies shield the investor’s identity. But, it turns out, the money came from a member of Israel’s Steinmetz family, which built a fortune as one of the world’s leading diamond traders.

A Kushner Companies spokeswoman and several Steinmetz representatives say Raz Steinmetz, 53, was behind the deals. His uncle, and the family’s most prominent figure, is the billionaire Beny Steinmetz, who is under scrutiny by law enforcement authorities in four countries. In the United States, federal prosecutors are investigating whether representatives of his firm bribed government officials in Guinea to secure a multibillion dollar mining concession. In Israel, Mr. Steinmetz was detained in December and questioned in a bribery and money laundering investigation. In Switzerland and Guinea, prosecutors have conducted similar inquiries.