9/11 Market Impact and the Put Options Anomaly

Historical Context

On September 11, 2001, terrorist attacks grounded the entire US aviation industry. The US stock market was closed for four days—the longest shutdown since the Great Depression. When the markets reopened on September 17, the Dow Jones Industrial Average suffered its largest one-day point drop in history at the time.

The Aviation Market Crash

Unsurprisingly, the aviation and insurance sectors were devastated. American Airlines (AMR) stock fell 39%, and United Airlines (UAL) stock plummeted 42% almost instantly when the markets reopened. The broader market also saw heavy selling, but the concentration of losses in the aviation sector was unprecedented.

The Options Anomaly Investigation

In the aftermath, financial regulators and the 9/11 Commission investigated anomalous trading patterns that occurred in the days just prior to the attacks. Between September 6 and September 10, an unusually high volume of Put Options were purchased on American Airlines and United Airlines. For example, the put-to-call ratio for UAL spiked dramatically, leading to widespread speculation of insider trading by individuals with foreknowledge.

Ultimately, the 9/11 Commission Report concluded that while the trading was highly anomalous, it was traced back to US-based institutional investors with no ties to al-Qaeda, who were hedging against a broader economic downturn. However, this event remains one of the most studied examples of extreme market reactions and the importance of trade monitoring.