The theory seemed to originate with the fringe right-wing site Shoebat.com, which is run by Walid Shoebat, a Palestinian-American who has claimed to be a former terrorist and "radicalized Muslim" before his conversion to Christianity.
In a lengthy post published Sunday, Shoebat and his son Theodore argue that Khizr Khan, who made an emotional appeal Thursday at the Democratic National Convention to vote for Hillary Clinton to honor his son, "wants to advance Sharia Law and bring Muslims into the United States."
The grammatically-challenged post concludes that Khan's academic work on Islamic law, along with his work in immigration legal services, means he is a jihadi. The writers also group Captain Khan in with so-called "Muslim martyrs" who they accuse of being double agents for the U.S. and Al-Qaeda.
"Is it likely that Khan’s son was killed before his Islamist mission was accomplished? Only another type of investigation will determine that," the blog reads.
That post came the same day that the Islamic State said Captain Khan died as an "apostate."
Longtime Trump adviser Roger Stone, who no longer has a formal role with the campaign but frequently acts as a campaign surrogate, was among the most notable voices pushing the claim, along with his favorite pet conspiracy theory about top Clinton aide Huma Abedin:
Mr. Khan more than an aggrieved father of a Muslim son- he's Muslim Brotherhood agent helping Hillary https://t.co/mJuUYw60nK
— Roger Stone (@RogerJStoneJr) August 1, 2016
But Stone wasn't alone in pushing the "Muslim Brotherhood" line.
Conservative columnist Cal Thomas tweeted the link and then deleted it after fierce blowback. It remained live on his Facebook page Monday morning, with the caption "there is always more to a story than the initial report."
New Hampshire state Rep. Al Baldasaro, a Trump delegate and co-chair of the real estate mogul's veterans group who infamously called for Clinton to be "shot for treason," advised his Twitter followers to "follow the money" on Khan:
— Rep Al Baldasaro (@Al_Baldasaro) August 1, 2016
In a blog post, Allen West offered his condolences to the Khans before chastising them for not using the DNC to denounce terrorism.
"You, Mr. Khan, had the chance to be the face and voice of a Muslim family that stood with the fundamental principles and values of liberty, freedom, and democracy upon which this Republic was founded," West wrote. "Mr. Khan, I grieve for the loss of your son. However, I grieve even more that you used his sacrifice and loss as nothing more than a damn politicized stunt."
Libertarian blogger Mike Cernovich, a vocal Trump supporter, compared Khan to Ahmed Mohamed, the Muslim teenager from Texas who was arrested for bringing a homemade clock to school that anti-Muslim activists insisted was "half a bomb":
Turns out Mr. Khan is reportedly a member of the Muslim Brotherhood. Another media hoax - like "clock boy."
— Mike Cernovich (@Cernovich) August 1, 2016
Conservative talk radio host Michael Savage simply tweeted the link to Shoebat:
— Michael Savage (@ASavageNation) August 1, 2016