The mystery of why Stephen Paddock smuggled an arsenal into a Las Vegas hotel room before firing into a crowd of concertgoers, killing dozens and injuring hundreds, has only deepened in the days since the massacre. His motives remain elusive, his beliefs unclear.

That absence of information has provided an opening for right-wing media personalities like Alex Jones of InfoWars to proffer their own theories. Their ideas, often based on scant evidence, fall in line with common right-wing tropes about liberal conspiracies, and millions of Americans are listening.

Mr. Jones — who has propagated conspiracy theories about the Sandy Hook Elementary School massacre in 2012 and the Sept. 11, 2001, terror attacks — has portrayed Mr. Paddock by turns as an agent of the Islamic State, a leftist activist, an anti-Trump radical and a possible stooge for a broader conspiracy intent on disrupting democracy.

“Could Stephen Paddock, the lone Vegas shooter, have been a patsy to kick off the left’s war with the right in the streets of America?” Mr. Jones wrote on his Facebook page last week. The post accompanied a video titled “Video Shows Second Shooter During Vegas Massacre,” which by Monday had been viewed about 1.1 million times.