He responded with his own tweet, saying, "This is a death threat, Representative. Clearly, you shouldn't own an AR-15—and neither should anyone else."
Cain replied, "You're a child Robert Francis."
In an interview with KTRK Friday, Cain said that his tweet was not meant to be a violent threat. He said his remark had a "come and take it" message, but he understands why Twitter, and others, saw it as threatening.
Cain told the station, however, that he believes it was overblown by the O'Rourke campaign and that due to threats he is now under protection from the Texas Department of Public Safety. Around 1 p.m. EST Friday, Cain told ABC News that federal authorities have not contacted him and that Twitter suspended him from tweeting for 10 hours.
The O'Rourke campaign is reporting the original tweet from Cain to the FBI, a campaign official told ABC News. His campaign has no further comment at this time.
O'Rourke has been particularly outspoken about banning military-style assault weapons and instituting a mandatory buy-back scheme since a man in an El Paso Walmart last month killed 22 people and injured dozens more. The suspect is thought to have used an AK-style rifle. El Paso is O'Rourke's hometown.
Most candidates in the Democratic debate on Thursday night made statements or commitments about regulating guns.