Back in December, the Intercontinental Exchange, owner of the New York Stock Exchange, received a Wells Notice from the SEC related to the company’s handling of a July 2015 glitch that halted trading on NYSE for nearly 4 hours and raised worries about the fragility of U.S. financial markets.

A Wells notice indicates the agency may bring an enforcement action against the company and allows defendants the opportunity to argue why the charges aren’t merited before they are filed.

The potential SEC enforcement action focuses on “how NYSE responded on 7/8/15 to the circumstances leading into the suspension of trading that day.” The outage stemmed from a faulty software upgrade to NYSE’s matching engine, a critical piece of technology that links up orders to buy and sell securities, the exchange operator said at the time. The glitch caused communication problems between traders and the exchange.

As a result, NYSE halted trading on its flagship exchange and its smaller NYSE MKT exchange at 11:32 a.m., a suspension that was not fully lifted until 3:10 p.m. that day. Trading on rival exchanges such as Nasdaq Inc. and Bats Global Markets Inc. continued while NYSE was down, meaning the outage was a nonevent for most investors.

But the incident sparked fears of what would have happened if NYSE had stayed down until 4:00 p.m., when the exchange runs closing auctions for NYSE-listed stocks that are used to determine their daily closing price - a key piece of financial information disseminated across the markets. In the wake of the incident, NYSE and Nasdaq agreed on a plan to act as each other’s backups in case one of the exchanges failed to hold a closing auction.

ICE CEO Jeffrey Sprecher told analysts that ICE disagrees with the SEC’s legal arguments, and characterized the event as a ‘technology outage’ He added, “It was very unfortunate and it was embarrassing and a black eye, but we don’t believe that it actually violated the law.”