In the days following the mass shooting at Tree of Life Synagogue in Pittsburgh, which left 11 people dead, law enforcement officials have found a disturbing trail of messages online in which the alleged shooter appeared to announce his attack.

Suspect Robert Bowers postedto a social network called "Gab," a 2-year-old site which had become a gathering place online for far-right figures after they were banned from more mainstream platforms like Facebook and Twitter. Gab went offline earlier this weekafter service provider GoDaddy terminated its contract with the site.

Homeland security expert Juliette Kayyem said that sites like Gab can play a role in radicalizing people.

"That is essentially the use of mass media and platforms to inspire and radicalize in ways that are utterly predictable: in other words, that someone would take the Soros stuff, view that as 'the Jews are letting in the caravan,' and then go into a synagogue," she continued, referring to a baseless right-wing conspiracy theory accusing Democrats of funding a migrant caravan 2,000 miles away from the U.S. border.

"Gab essentially was the place for antisemites and racists to spew," she said. "This is obviously where the synagogue killer found like-minded people."

Kayyem said the conspiracy theories trafficked by people on sites like Gab have found willing audiences in political figures, including the president, who amplify their reach.

"What we're seeing is a form of inspiration or radicalization by our leadership, and it's not just Donald Trump," Kayyem said. "It's the retweeting by senior senators, it's the Fox News network."