A tweet by conservative columnist Erik Rush — that all Muslims should be killed in response to the Boston Marathon bombings — has set off a firestorm of debate on Twitter.

The hashtag #muslims was trending as pro- and anti-Islamic voices weighed in.

In what was widely described as a "joke," Rush tweeted:

Everybody do the National Security Ankle Grab! Let's bring more Saudis in without screening them! C'mon! #bostonmarathon — Erik Rush (@erikrush) April 15, 2013

When a responder tweeted, "Are you ALREADY BLAMING MUSLIMS??" Rush responded:

"Yes, they're evil. Let's kill them all."

He later tweeted that he was being sarcastic.

However, when a predictable flood of abuse, shocked disgust and the occasional encouragement came, Rush appeared defiant, retweeting the more profane of the responses.

Some reports referred to Rush as a Fox News contributor. Carly Shanahan of Fox News told GlobalPost on Tuesday: "Rush is not a Fox News contributor, he is only a guest and not even a regular guest of the network - he has been on a total of five times in the last twelve months."

Here is a cross section that we found representative of the reaction.