Shares of Citigroup (C) - Get Report were hit extremely hard on Friday. The stock fell over 9% on its heaviest volume since March of 2014. Overall, the money center banks were sold aggressively in the wake of the Brexit results. The Money Center Bank Index itself fell over 9%.

Citigroup was one of the most severely damaged and could quite possibly be headed much lower.

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Back in mid-May, Citigroup settled in near a key support zone after peaking in late April. The steady pullback that began just after shares failed to clear the $48 area created a very low-risk entry opportunity for investors. Citigroup mounted a solid rebound off the $43.50 area before once again stalling just below its declining 200-day moving average. As this month began, Citigroup had lost all its momentum and was rolling over. Two weeks in and the stock was trading below the May low before managing a sharp bounce as Brexit fears began to ease.

The Friday flush has put a tremendous amount of pressure on Citigroup investors. This massive swing of downside momentum will carry shares lower. Citigroup bulls should keep a close eye on the key support zone that held the January and March lows. If this area, between $39.50 and $39, is clearly taken out, the stock will have little else in the way of support until the 52-week lows at $34.50 are retested. If shares reach this major low on easing volume, coupled with an oversold moving average convergence/divergence reading, a very low-risk entry opportunity will be at hand.

From a fundamental perspective, Citigroup is a holding in Jim Cramer's Action Alerts PLUS charitable portfolio. Cramer and Research Director Jack Mohr wrote on Friday:

The stock got hammered this week, with the large majority of the decline coming on Friday following the Brexit vote. Financials were pressured, in particular, as rates declined and the potential for a near-term rate hike all but disappeared. Prior to the vote, Citi, among other banks, received good news when it was told it met capital requirements required by the Fed's stress test. While this is a minor win, the big test comes next week when the CCAR results are announced. As we have stated, we like Citi for its solid regulatory standing, which should bode well for its CCAR prospects. In the most recent test, Citigroup improved its Tier 1 common equity ratio to 9.2% from 6.8% and its Tier 1 leverage ratio to 6.9% from 4.6%. For now, we are maintaining our rating and keeping our $55 price target, but note that it is under review given the uncertain economic implications of Brexit.

Disclosure: This article is commentary by an independent contributor. At the time of publication, the author was long C.