At the height of the recession when dealmaking was just a memory, a small investment group was quietly amassing a diverse portfolio of heavily discounted real estate across South Florida.

Called 13th Floor Investments LLC, the company bought below replacement cost, added value to the often unfinished projects and resold them for hefty profits. The deals were complex — often involving foreclosures and the purchase of notes — and risky, said 13th Floor managing principal Arnaud Karsenti.