“I will proceed to slaughter and murder any doctor, patient, or visitor I see in the area and I will not back down,” the iFunny user “awarded” — which the authorities linked to Mr. Sheikh — wrote on the website on Aug. 13. “Consider this a warning for anyone visiting.”

In a separate post, the user issued further clarification: “To all the F.B.I. agents in this app, I am NOT a satirical account. I post what I mean, and I WILL carry out what I post.”

The user also lamented the arrest this month of the Ohio man, Justin Olsen, 18, who was accused last week of supporting mass shootings and targeting federal agents and Planned Parenthood in posts on iFunny. The authorities seized more than a dozen rifles and about 10,000 rounds of ammunition from Mr. Olsen’s residence, according to a court filing.