truther

1. (US, colloquial) Someone who tells the truth.Examples: "A liar can't begin to start the riots a truther can."UScolloquial

2. (colloquial, chiefly US) Someone who believes that the US government was responsible for the 9/11 attacks.Examples: "[signed] 9/11 Truther"; "Therefore, virtually every press release or statement about 9/11 should include a reference to the poll to show people that many Americans are 9/11 truthers who scoff at the 9/11 Commission as a fraud and that demand a real 9/11 investigation."; "The Iranian president is even a "truther," insisting that there was a hidden hand behind Sept. 11, 2001. "Could it be planned and executed without coordination with intelligence and security services — or their extensive infiltration?" he mused in the letter to Bush."UScolloquial

3. (by extension) A conspiracy theorist who does not believe the accepted story about some significant event, fact, etc.Examples: "For instance, "truthers" who believe in a US government conspiracy to destroy the pentagon"; "Rather, believers of this conspiracy theory (known as Sandy Hook “truthers”) propose that the entire event might have been staged by Homeland Security and FEMA"; "I am calling your attention to it because I think that we JFK truthers may be able to use this film to our advantage."broadly