Fans of This Is Us are bracing themselves for more reveals about Jack's (Milo Ventimiglia) death reveal, and after learning the first piece of the puzzle-the when for his death-many theories have begun to float around about what leads to the Pearson Patriarch's untimely end.

Here are four theories for how Papa Jack could die:

9/11 Terrorist Attacks:

A popular theory that has begun to circulate now that we know the Pearson triplets were teens when their father died has been that Jack was somehow killed in the 9/11 terrorist attacks. The theory has been supported by the fact that both Kevin (Justin Hartley) and Kate (Chrissy Metz) in the present day have issues and fears about flying on planes, as well as the fact that Randall (Sterling K Brown) and Rebecca (Mandy Moore) both live in New York City suburbs, meaning it's likely Jack could have gotten a city job at some point. However, since the kids were all born in 1980, it would make them 21 in 2001, meaning Jack's death was likely before then since they were teenagers when he died.

General Plane Crash:

Another theory is that Jack flew somewhere and died in a plane crash. This could also be evidenced by the fact that Kevin and Kate are both scared of flying, as well as Kate thinking the turbulence she experienced on her flight to the East Coast was a sign of her needing to make a rash decision about how to lose the weight. In addition, Kevin's earlier confession that he destroyed all the model planes he and Jack had made when he was a kid also seems to indicate a possible issue with planes.

Alzheimer's:

In the 'Best Washing Machine In The Whole World' episode, Jack seemed forgetful about a lot of things-which has some wondering if maybe he suffered from early Alzheimer's. However, the tragedy of his death seems to be that it happened suddenly-meaning if he had died from Alzheimer's complications that the family would have had more time to prepare for the loss.

Heart Failure:

The theory that seems most likely is that something happened to Jack's heart which caused him to die suddenly. Kate is the one who seemed to be the most affected by his death, which has some wondering if she was alone with her father, potentially even watching a Steelers game, when he died. In addition, it would also explain how her weight is tied to the tragedy, and also why she was so quick to have Toby (Chris Sullivan) try for surgery to repair his own heart, and was disheartened when he initially rejected the idea.

This Is Us airs Tuesdays at 9 p.m. on NBC.

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