INDIANAPOLIS (AP) — Two new Indiana University studies say downtown Indianapolis has become an economic engine for the entire state thanks to two decades of investment in the state's capital.

The studies commissioned by Indianapolis Downtown Inc. say the city's downtown district has seen $9 billion in investments since 1990, including Lucas Oil Stadium, an expanded convention center and new housing developments that have given the city a total of 4,700 rental units.

The Indianapolis Star reports the investment have made the downtown area a desirable residents for a growing numbers of professionals.

But one of the studies says one question is whether the city's downtown can support the 3,500 new apartments expected to be built from 2014 to 2017, without causing a large jump in the vacancy rate.