SAN FRANCISCO — 8chan is back.

The anonymous message board began flickering back online on Saturday and was fully visible and available on Monday, three months after it had gone dark.

The site, which has served as a megaphone for violent extremists, was knocked offline in August after several tech companies refused to provide it with critical services such as a functioning web address. At the time, the tech companies said they would not work with 8chan because it provided mass killers with a place to air and spread their violent and often racist messages.

The shooting at a Walmart in El Paso in early August, along with attacks at mosques in Christchurch, New Zealand, and at a synagogue in Poway, Calif., this year were all announced on 8chan before they began. The attackers posted screeds to a section of 8chan that was ostensibly dedicated to politics, and does not appear on the new site.

After the El Paso shooting, one of the 8chan founders, Fredrick Brennan, said, “Shut the site down.” In September, the site’s owner, Jim Watkins, testified in front of Congress about 8chan’s operation and policies. The site had been operating out of the Philippines.