"For too long the kuffar (non-Muslims) have spit in our faces and trampled our rights. This cannot continue. I cannot speak of anything. Say your dua (prayers), sleep, and watch the news tomorrow. It will only be the beginning, insha'Allah."
Authorities were able to track Grimsley down and arrest him on the same day he posted by asking Whisper for his IP address and GPS coordinates. After feds got his IP address from Whisper, they were able to get Time Warner Cable to match it with a name, a physical address and a phone number.
The app is supposed to be completely anonymous, but you'll have to opt out of location services if you don't want it to receive your geographical coordinates -- Grimsley might have left his active. It's worth noting, however, that a 2014 report by The Guardian accused the company of retaining posts and user information in a searchable database. Whisper CEO Michael Heyward denied those accusations.
Cops found an AK-47, four 30-round magazines and about 340 rounds of ammo in a search of his home. But they also found a Facebook conversation on his laptop that makes his true intentions unclear. In it, he said he "yell(s) all the time about jihad" due to "too much CSGO [Counter Strike: Global Offensive] and nasheeds [Islamic videos of vocal music]." He even jokingly asked the other person: "the FBI will understand I was only reading [an ISIS magazine] for the pictures, right?" Authorities couldn't connect him with a terror group.
Feds have charged Grimsley with "transmitting a threat to injure a person" as well as with cyberstalking. If he's convicted, he could end up spending up to five years in prison and paying up to $250,000.