Houston-based lawyer Charles Cotton, listed as a national NRA board member on the gun lobby's website, made the comments in an online chat room he administers called texaschlforum.com, a discussion board devoted to gun rights and firearms issues.
In an online thread about Wednesday night's mass shooting at the Emanuel African Methodist Church in Charleston, Cotton said that one of the nine people slain, church pastor and Democratic state Senator Clementa Pinckney, had voted against legislation in 2011 that would have allowed concealed possession of handguns in restaurants, day-care centers and churches.
"Eight of his church members who might be alive if he had expressly allowed members to carry handguns in church are dead," Cotton wrote.
The comments, since deleted from the online forum but ricocheted across social media in cached versions, triggered a torrent of online condemnation, with hundreds of posts bombarding Cotton's Twitter handle branding him a "monster" and "insensitive."
The online Christian social justice organization Faithful America launched an online campaign seeking 15,000 signatures for a petition demanding that the NRA remove Cotton from its board and apologize for his remarks.
Cotton did not respond to requests from Reuters seeking comment.
But The Washington Post quoted him on Friday as saying that the online discussion that generated the controversy was about the pros and cons of "gun-free zones," and his comments were made as a private citizen.
"It's my opinion that there should not be any gun-free zones in schools or churches or anywhere else. If we look at mass shootings that occur, most happen in gun-free zones," he told the newspaper.
Authorities have said the suspected gunman in the Charleston shooting, Dylann Roof, 21, who is white, spent an hour in Bible study with black parishioners at the nearly 200-year-old church before opening fire on them. He was charged with nine counts of murder and a weapons offense on Friday, a day after his arrest in North Carolina.
The U.S. Justice Department said it is investigating the church attack as both a hate crime and potential act of terrorism.