John P. Carlin is the new head of the Justice Department’s National Security Division. (Michael S. Williamson/The Washington Post)

Almost every morning at 9, the nation’s senior law enforcement officials meet in a top-secret room at FBI headquarters for a briefing on terrorist threats. Attorney General Eric H. Holder Jr. attends, as does FBI Director James B. Comey. And so does a guy named John P. Carlin.

Carlin is not exactly a household name in Washington or beyond. But the new assistant attorney general in charge of the Justice Department’s National Security Division is privy to the nation’s most classified secrets about government surveillance, counterterrorism operations, targeted drone killings, cyberhacking and counterespionage.

A soft-spoken 40-year-old dubbed “the Boy Scout” by one Justice official, Carlin fits the mold of a national security mandarin: He doesn’t want to talk publicly about anything, including himself.

Carlin’s focus on national security was shaped by the Sept. 11, 2001, terrorist attacks. A Harvard Law School graduate, Carlin was set to start work as a prosecutor in the U.S. Attorney’s Office in Washington when the planes hit the World Trade Center towers.

“I’m from New York and my family is from New York,” Carlin said in an interview. “My father was in a subway underneath the towers headed to a meeting, and my brother-in-law worked across the street and watched people jump out of the building. My wife was in the city at the time. We were all checking the phones to make sure everyone was safe. For us, they were. For too many people, they were not. ”

The National Security Division was born in the aftermath of the terrorist attacks, created in 2006 as one of the government’s post-9/11 reforms. The idea behind the NSD, the first new litigating division of the Justice Department in 50 years, was to create a single unit for terrorism, espionage, intelligence and criminal lawyers to work with U.S. attorney’s offices and the FBI to help prevent terrorist attacks.

Carlin, who oversees 220 lawyers, has four phones on his desk in the Sensitive Compartmented Information Facility (SCIF), where no reporters are allowed. All but one are for top-secret-level conversations with other agencies.

The NSD was at the forefront of the controversy over U.S. government surveillance in light of the leaks by former National Security Agency contractor Edward Snow­den. Carlin’s attorneys conduct legal oversight of FBI national security investigations and go before the Foreign Intelligence Surveillance Court to ask for authority to collect communications.

Lawyers at the National Security Division also work behind the scenes on the prosecutions of terrorists. Before any U.S. attorney brings a terrorism case, it must be approved by Carlin’s division.

Some national security lawyers privately expressed concern about Carlin’s rise through the ranks to this key post, citing a lack of experience as a national security prosecutor. But Kenneth L. Wainstein, the first assistant attorney general for national security, said that “he’s got all the qualities you want. John has always been a top-notch prosecutor, has deep experience in cyber matters and spent several years at the FBI and Justice Department doing national security work.”

Carlin and his attorneys helped with the recent terrorism prosecution of Sulaiman Abu Ghaith, the son-in-law of Osama bin Laden. But after the guilty verdict came in, Carlin was not on stage for the news conference with Holder and Preet Bharara, U.S. attorney for the Southern District of New York.

“He's not a press guy,” Bharara said. “He doesn’t like to call attention to himself. John’s a guy who just tries to do his job.” Carlin takes his job, but not himself, seriously, he added.

“John will always see the irony and absurdity in a situation, which is much appreciated,” Bharara said. “He has a sense of humor, and it’s a bit dry and a bit sarcastic.”

Carlin was reportedly not Holder’s first choice for the job. But Holder’s former national security aide, Amy Jeffress, was passed over by the White House, where Carlin’s advocates included Kathryn Ruemmler, White House counsel, and Lisa Monaco, assistant to the president for homeland security and counterterrorism.

Ruemmler, Monaco and Carlin were colleagues in the District of Columbia’s U.S. Attorney’s Office. Carlin prosecuted violent crime, fraud and public corruption and developed an expertise in cybercrime. After Monaco left the U.S. Attorney’s Office to become chief of staff for FBI Director Robert S. Mueller III, she recruited Carlin in 2007, because the Bureau needed expertise on the emerging cyberthreat.

“John is whip-smart,” Monaco said. “He just has incredible judgment when you’re dealing with very intense and important and complicated operational matters.” She said he walked to work in the dark every day from his home in the District, arriving for a 7 a.m. briefing with the FBI director. He also walked home late at night in the dark.

Monaco next became head of the National Security Division at Justice, and says she “stole” Carlin from Mueller to be her chief of staff and then deputy specializing in cyberespionage and cyberterrorism.

The son of a New York lawyer and a poet who teaches literature and creative writing, Carlin met his wife, Sarah Newman, an art historian, when he was an undergraduate at Williams College. He wrote his college thesis on Shakespeare as a political philosopher. His favorite plays are the Henry tetralogy, but he bemoans the fact that he has no time to see “Henry IV” at the Shakespeare Theatre Company.

The “light of his life,” one friend said, is Carlin's nearly 4-year-old daughter. He turns to the children’s story “The Three Pigs” to explain why he works so late.

“She was worried about the bad wolf,” Carlin said. “I said I make sure there are no wolves.”

Carol D. Leonnig and Julie Tate contributed to this report.