Sullivan rejected a figure in a recently published book by Ronald Kessler, "In the President's Secret Service." Kessler says the threat has increased 400 percent since Obama, the nation's first African-American president, took office.
In October, Kessler told CNN he stands by the figure even though a law enforcement source had told CNN it's false.
"There is about 3,000 threats a year under President Bush and now there are about 12,000. Of course, most of them are not credible, but they all have to be checked out," Kessler told CNN's "The Situation Room" in an interview with Wolf Blitzer in October.
He said he got the figure "originally from another agency, but I went over it with the Secret Service."
Sullivan, at a Congressional hearing about how a couple allegedly crashed a state dinner, brought up the 400 percent figure and rejected it, adding that he is "not sure where that number came from."