"We now have a," Chertoff said on the sidelines of a meeting of International Association of the Chiefs of Police.
"They can train themselves over the Internet. They never have to necessarily go to the training camp or speak with anybody else and that diffusion of a combination of hatred and technical skills in things like bomb-making is a dangerous combination," Chertoff said. "Those are the kind of terrorists that."
Chertoff pointed to the July 7, 2005, attacks on London's transit system, which killed 56 people, as an example a home-grown threat.
To help gather intelligence on possible home-grown attackers, Chertoff said Homeland Security wouldwhere they would work with local police agencies.
By the end of the next fiscal year, he said the department aims to up that to 35 staffers.