One clearinghouse for the campaign, parts of which were earlier reported by Wired, is a website that appears to have been registered just three hours after Mr. Kirk was killed. The site includes a tip line for submissions and includes photos of the people who disparaged him or cheered his death, along with personal information like their email addresses, place of residence and employer.

As of Friday afternoon, 41 people had been named on the site. Its anonymous creator posted that the site had received almost 20,000 submissions and said the names would be put in a searchable public database of “radical activists calling for violence,” while also denying that it was “a doxxing website.”