Feb. 27, 2013 -- As the nation's top law-enforcement officer, Eric Holder is briefed daily on terrorists threats. He attends meetings in the White House situation room, and he decides when to ask judges for the death penalty. At night, Holder says, he worries about terrorist threats.
But his worst day on the job came Dec. 20, when he traveled to Newtown, Conn., to meet with first responders and visit the crime scene where gunman Adam Lanza had killed 20 children and six adults with a high-powered rifle at Sandy Hook Elementary School six days earlier.
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"I was on a trip out of town. [FBI Director] Bob Mueller called me and said that there had been a really horrific shooting in Connecticut. And he said, 'It's really bad, Eric. It's really, really bad,'" Holder said Wednesday in a wide-ranging exclusive interview with ABC's Pierre Thomas.
Discussing his personal experience with Newtown at length, Holder detailed how he first heard of the shooting. When it happened, Holder was in Tulsa, Okla., for a ceremony honoring the new U.S. attorney for the state's Northern District.
"We turned on the news to get a sense of what had happened, and Bob called back and started to give me some numbers and then said, 'And it looks like it's little kids,'" Holder said. "I understood at that point, given both the numbers and who the victims were, that we were dealing with something unlike anything we'd ever seen before."
Walking through the crime scene, Holder said, was the most difficult moment of his career.
"The worst day I've had as attorney general was the day that I went up to Sandy Hook to say thanks to the first responders and to the people who were the first on the scene," Holder said.
"And I have to tell you that walking through Sandy Hook Elementary School and going into those classrooms and seeing the caked blood, seeing the crime scene photos of these little angels was the most difficult thing that I've ever had to do in my professional life," Holder said, describing how both he and the first responders struggled unsuccessfully to hold back tears.
"There were tears from me, from the first responders, from the crime scene search officers," Holder said. The attorney general "had a pretty emotional ride, after I left the school, going back to the airport."
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The attorney general helped the administration draft its set of gun-control proposals as a part of Vice President Joe Biden's working group. That effort has stalled as congressional Republicans have rejected many of the proposed measures, including a reinstated assault-weapons ban and limits on magazine capacity.
In his interview with ABC News, Holder reiterated his bosses' call for new gun-control measures. The FBI, he said, is looking for ways to sooner identify potential mass shooters.
"I think what we as an administration, we as a nation have said is, Enough is enough, that there are limits to how far we should go and that we should come up with really common-sense, responsible ways in which we deal with this problem. And that's what we have proposed," Holder said.
"We have done, I think, a pretty good job in identifying those who might be susceptible to terrorist entreaties and become homegrown violent extremists. I think we need to apply some of those techniques to see if we can pick out ahead of time who these potential mass killers are," Holder said. "So it is something that we're working on."