CHICAGO — The US Securities and Exchange Commission(SEC) on Wednesday postponed a decision on whether to allow a Chinese investor group to buy the Chicago Stock Exchange.

An investor group led by Chongqing Casin Enterprise Group (Casin Group) has been seeking SEC approval of the deal since late 2016, with numerous letters challenging the deal.

Among the latest in July, 11 US House of Representatives urged the SEC to reject the proposed acquisition, calling it "a threat to the US financial security and Americans' faith in our national financial market infrastructure."

The SEC was supposed to rule on the $30 million sale Wednesday. Instead, the regulatory body said SEC commissioners would make the final decision in the coming weeks, a move that overrides a recommendation by commission staff.

"We are confident that, upon further review by the commissioners, they will also conclude that this transaction is consistent with the Exchange Act," a Chicago Stock Exchange spokesman said.

Casin Group, headquartered in China's southwestern metropolis of Chongqing, is a leading diversified holding company with investments in financial services, real estate and environmental services.

The 134-year-old Chicago Stock Exchange, where all trading is done electronically without a live trading floor, now handles about 0.5 percent of US stock trading.