Some other terrorist attackers have been "self-radicalized," including the pair who killed 14 people in December in San Bernardino, Calif., who also proclaimed allegiance to the Islamic State, but apparently had no contact with the group.
Since then, federal authorities have said the couple was "self-radicalized" in part by reading radical Islamic literature online.
Officials cautioned that the new evidence could point to self-radicalization. "Were they directed or self-radicalized?" the source said.
"Right now, there are unfortunately young people in this country that are self-radicalized, and what vehicle is doing the self-radicalization? It's the internet.
Authorities said the two shooters in that case, who gunned down 14 people in what was declared a terrorist attack, also appeared to be self-radicalized and not directed by any outside group.
Two self-radicalized Russian immigrant brothers are suspected in the attack; one was slain in a shootout.
Still, he acknowledged that lone-wolf attacks, in which perpetrators become "self-radicalized" in the mode of Malik and Farook, are among the most difficult for U.S. intelligence agencies to detect and prevent.
The Paris attack has challenged the notion that self-radicalized loners and cells are the greatest threat to the US and Europe.
The FBI is constructing a detailed timeline of the still-hazy years between when Syed Rizwan Farook and Tashfeen Malik became self-radicalized on the Internet and when they launched the deadly attack in San Bernardino.
But spectacular shootings in San Bernardino and Orlando, and an attempted attack in New York and New Jersey, have recently focused attention on self-radicalized Muslims and immigration.
Most importantly, self-radicalized terrorists lack the organizational infrastructure and strategic guidance that large-scale attacks like 9/11 require.
