Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Brussels, Belgium (CNN) In grainy images from surveillance footage, a man wearing light-colored clothes and a hat pushes a baggage cart through the airport. It's one key piece of evidence authorities are looking at as they search for suspects after two explosions at the Brussels airport and another at a busy subway station in the Belgian capital Tuesday killed at least 30 people and wounded 230 others. Brussels attack investigation: 1 person arrested ISIS claimed responsibility for the coordinated attacks, but authorities said it's too soon to say for sure whether the terror group was behind the blasts. So far, police have released photos of three men they say are suspects tied to the airport attack, standing side-by-side.Authorities comb through a building where a taxi driver says he picked up the suspectsProsecutor: Of three men pushing airport luggage carts in photos, two are probable suicide attackers and one is at largeTwo explosions rock Brussels Airport, another rips through a subway station in the Belgian capital 
 Question:Investigators found a nail bomb, chemical products and an _ flag during a house search in the northeast Brussels neighborhood of Schaerbeek, Belgium's federal prosecutor said in a statement.

ISIS