On November 15, 1979, American Airlines Flight 444, a Boeing 727 flying from Chicago to Washington, D.C., was attacked by Ted Kaczynski, the man who would later become infamous as the Unabomber. Kaczynski had mailed a pipe bomb set to detonate at a specific altitude, and it partially exploded in the cargo hold, causing a sudden loss of cabin pressure and filling the plane with thick smoke. The pilots were forced to make an emergency landing at Dulles International Airport, where twelve passengers were treated for smoke inhalation. The FBI recovered the relatively intact bomb and linked it to two earlier explosions at Northwestern University, leading them to dub the unknown culprit the "Unabomber" (for University and Airline Bomber). This attack was a pivotal early event in one of the FBI's longest and most expensive manhunts, which finally ended with Kaczynski's arrest in 1996.