Wall Street's top regulator has rejected a Chinese-tied investor group's bid to acquire Chicago Stock Exchange Inc.

The SEC officially rejected the proposed acquisition Feb. 15, marking what could be the end of a heated two-year fight in which critics of the deal speculated that the proposed acquisition would open U.S. markets to threats from foreign entities.

The decision to block the bid came about six months after the SEC's staff recommended that the transaction be approved, a decision that was ultimately overridden by the SEC's commission, which made the final decision to block the transaction.

"The Commission's review of the information before it — including, but not limited to, the staff's experience in gathering information to assess the proposed rule change — leads us to conclude that [Chicago Stock Exchange] has not met its burden to demonstrate that the proposed rule change is consistent with the [Securities] Exchange Act," the SEC wrote in a Feb. 15 filing.

Announced in February 2016, the transaction would have made Chicago Stock Exchange the first U.S. exchange operator with owners tied to China. A mix of U.S.- and China-based companies made up the investor group, but concerns over the bid reportedly led some of them to drop out in late 2017.

A spokesman for Chicago Stock Exchange declined to comment.

The exchange operator planned to use the acquisition as a foothold in the increasingly competitive exchange space as officials hoped to establish the Chicago Stock Exchange as a front door for Chinese companies to go public in the U.S.

But the proposed transaction's lifespan was rife with lawmakers and other industry players airing concerns that it could expose the U.S. markets to national security threats, increase the odds of cybersecurity incidents and risk compromising investors' personally identifiable information.

Chicago Stock Exchange officials combated criticisms that came despite a December 2016 approval from the U.S. Committee on Foreign Investment in the United States, a federal agency that reviews such deals.

In prior comment letters, Chicago Stock Exchange CEO John Kerin wrote that some concerns raised over the merger were "blatantly false and misleading."

Most recently, the exchange operator took aim at the SEC itself. In January, Chicago Stock Exchange Executive Vice President and General Counsel James Ongena wrote in a letter to the SEC that the regulator had violated provisions of the Securities Exchange Act as well as its own rules by delaying a ruling on the proposed acquisition.

A separate group dubbed Exchange Capital LLC has also been waiting for the SEC's decision on the potential transaction.

The group has reportedly emerged as a potential suitor for the exchange operator, with its bid marketed as including "all American" money, according to a September 2017 report from The Wall Street Journal.