The $508.2 million sale of the Detroit Renaissance Center, which had been expected to have been completed Sept. 30, has been postponed because of continued problems in locating investors, according to Richard Routh, a spokesman for Ford Motor Land Development.

Citing difficulties beyond his company's control, Routh refused to set a new deadline for closing.

This is the second time the sale's closing date has been moved. The closing originally was set for June 30 but then was shifted to Sept. 30.

The announcement of the second postponement is causing particular anxiety in Detroit because Henry Ford II said during the spring that the alternative to the sale "was to go broke."

The RenCen, four 39-story office towers and the 71-story Westin Hotel, was once the symbol of Detroit's rejuvenation. But since the center opened in 1977 it has lost $140 million and has been unable to pay many of its bills.

The hotel is only 60 percent occupied, much lower than originally anticipated. The retail segment of the center is only 58 percent occupied.

The complex's owners, including Ford Land and 50 other, mostly-auto-related companies, have been trying to sell the Renaissance Center for more than a year.

Although occupancy in the office towers is running at a healthy 96 percent, leasing officials concede that they are getting less than the market rate for the floor space. With many leases coming up for renewal, some are fearing that raising the rental rates will force some businesses and professionals to leave the office towers.

Chicago real estate magnate Theodore Netzky, who organized the RenCen sale and was to purchase it, could not be reached for comment.

To raise $77.9 million of the selling price, Netzky is trying to sell 200 shares of RenCen Ltd. Each share of RenCen Ltd. represents ownership in the Renaissance Center.

A spokesman at Ford declined to indicate how many of the shares have been sold. But it is the sale of these limited partnership units which is delaying the final sale.

Each share costs $389,500 and is supposed to generate tax savings of $1 million over 10 years from the center's anticipated losses.

Ever since the center opened with a nationwide fanfare of publicity, Detroit residents have been criticizing it. They point out that its isolated location behind a pair of concrete slabs hide it from the city it was geared toward helping.