The social-media platform favored by Robert Bowers, accused in the Tree of Life synagogue massacre, is the subject of a civil investigation by the office of Attorney General Josh Shapiro, according to a subpoena made public on Twitter late Wednesday.

The subpoena to Epik.com, a Seattle firm and Gab.com's new web hosting and domain registration provider, demands documents describing Epik's relationship with the social-media site. It also seeks any complaints Epik has received about Gab. The subpoena indicates that it is part of "an ongoing civil investigation," and is signed by Timothy R. Murphy, a deputy attorney general. It does not mention Bowers.

"The Pennsylvania Office of Attorney General has issued a subpoena," office spokesperson Joe Grace confirmed in an email. "We cannot comment further on an ongoing investigation."

Bowers, 46, of the Pittsburgh suburb of Baldwin, is charged with 44 federal counts in the attack, which authorities have designated a hate crime, as well as 36 counts in the state court system, including 11 homicides and ethnic intimidation, stemming from the Oct. 27 massacre.

During October, according to archives posts on Gab, someone who appears to be Bowers was posting and reposting anti-Semitic and anti-immigrant messages.

Little more than a week before the massacre, he began posting on the "mass migration" of immigrants and refugees. At around the same time, he zoomed in on HIAS, formerly the Hebrew Immigrant Aid Society, which helps refugees of all faiths to settle in the United States, after they are vetted and approved by federal authorities. HIAS's local partners include one of the Tree of Life congregations. Referring to HIAS, he posted: "You like to bring in hostile invaders to dwell among us?"

And finally, just before the attack, he posted: "HIAS likes to bring invaders in that kill our people. I can't sit by and watch my people get slaughtered. Screw your optics. I'm going in."

Gab was launched in August 2016 by Andrew Torba and Ekrem Buyukkaya, a resident of Turkey, in response to what they called Twitter's policy of "banning outspoken conservatives" or "compromising" their communications.

"Gab very, very quickly became a haven for far-right extremism, white supremacists, neo-Nazis, and so forth," said Peter Simi, an associate professor of sociology at Chapman University in Orange, Calif., and author of the forthcoming book American Swastika: Inside the White Power Movement's Hidden Spaces of Hate.

Gab was largely offline during the week immediately after the shooting, when several vendors stopped doing business with it. It came back online on Nov. 4, when Seattle-based Epik.com agreed to host it.

In a release explaining Epik's decision to back Gab, Epik's chief executive wrote that Gab has "a duty to monitor and lightly curate, keeping content within the bounds of the law." Torba, Gab's CEO, meanwhile, has been posting messages of tolerance and the hashtag #NoMoreHateOnGab writing that "it's absolutely imperative that we have diversity and love on our platform."