DETROIT-Title Source, a firm owned by Rock Ventures owner Dan Gilbert, is moving 1,500 employees to fill 330,000 square feet at the First National Building downtown. The move boosts the building’s occupancy to 93%.

Gilbert purchased the 800,000-square-foot building, and a number of vacant and nearly-vacant buildings downtown, within the past 24 months, and has engaged in a massive redevelopment and lease-up program. The effort began in August 2010, when Gilbert announced he would move his Quicken Loans headquarters from the Livonia, MI suburb to the downtown.

Gilbert has since led a plan for Detroit’s resurgence to get people working and living downtown again, started by the General Motors Corp. move downtown that stalled during the recession. By early 2013, the number of Gilbert’s employees working in downtown Detroit will surpass 6,500. According to a Rock Ventures statement, more than 60 companies have moved into company-owned buildings, including the Madison building, Chase Tower and the Dime building.