President Obama receives an update on the explosions that occurred in Boston, in the Oval Office on Monday. Seated from left, are: Homeland Security Secretary Janet Napolitano, Deputy National Security Advisor Tony Blinken, National Security Advisor to the Vice President Jake Sullivan, Attorney General Eric Holder, Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco, Chief of Staff Denis McDonough and FBI Director Robert Mueller. (HANDOUT/REUTERS)

The day after the Boston bombings, President Obama met in the Oval Office with his most trusted counterterrorism advisers. On a taupe couch, wedged between FBI Director Robert S. Mueller III and Attorney General Eric H. Holder Jr., a solemn-looking woman tightened her focus on the president.

It was Lisa Monaco, Obama’s 45-year-old counterterrorism adviser, who only several weeks after starting her new job is advising and briefing the president on the first large-scale terrorist attack in the United States since Sept. 11, 2001.

That alone would put enormous pressure on a new presidential adviser. But the bombing of the Boston Marathon is also personal for Monaco, who was raised in the the Boston suburb of Newton, where her parents still live. She and her family used to line up on the marathon route to watch one of her brothers run the race.

The personal contact with Obama and her handling of the response to the attack is also a high-stakes tryout for Monaco. She is one of the leading candidates to take over the helm of the FBI when Mueller steps down in five months, according to law enforcement sources with knowledge of the selection process. If chosen, she would be the first woman to head the bureau in its 105-year history.

Monaco has years of experience in the national security arena as a counsel to former Attorney General Janet Reno, as Mueller’s chief of staff at the FBI and as Holder’s assistant attorney general for national security.

As she is compared to another leading contender for the job, James B. Comey, a seasoned prosecutor and former deputy attorney general, current and former law enforcement officials say the question is: Does Monaco have what it takes to head one of the largest and most famous investigative agencies in the world?

Her performance this week could be a determining factor. “It’s hers to win,” said one former law enforcement official, speaking on the condition of anonymity to discuss the matter candidly. “It’s like an extended job interview.”

When she was at the Justice Department and since she has been at the White House, Monaco has declined repeated requests for interviews centering on her. Those who know her say she values her privacy and is self-deprecating. Unmarried and without children, she works around the clock but keeps a close-knit group of loyal friends.

“She’s deeply committed to the law enforcement mission,” said White House Counsel Kathryn Ruemmler, a close friend. “She’s funny, low-key and not a self-promoter. She’s the kind of loyal friend you would want with you in the foxhole.”

Monaco attended the all-girl’s Winsor School in Boston and later Harvard University and the University of Chicago Law School. At the 25th reunion of her Winsor School class in June 2011, Juliet Eastland approached her former high school classmate looking for some gossip. She wasn’t after the usual dish.

“After a few drinks, I said, ‘Tell me about the bin Laden raid,’ ” Eastland recalled. She said Monaco adeptly divulged nothing, responding that “credit was due to a lot of people.”

After clerking for a judge on the U.S. Court of Appeals for the Third Circuit in 1998, Monaco went to the Justice Department to serve as a counsel to Reno. Three years later, she joined the U.S. Attorney’s Office for the District of Columbia.

She served on the Justice Department’s Enron Task Force as the co-lead trial counsel in the prosecution of five former Enron executives. As an assistant U.S. attorney, she joined the FBI on a detail and then became Mueller’s counsel and chief of staff.

Monaco then served as Holder’s top official overseeing the National Security Division for nearly two years.

“From the moment I laid eyes on her, it was clear that people in position of authority — Mueller, Reno, Holder and the top of law enforcement — relied on her enormously,” said Robert Raben, an assistant attorney general in the Clinton administration and now a Washington lobbyist.

“She’s one of those people who is able to absorb a body of complicated law and facts and then when it’s her turn and her place, give an amazingly clear, candid, precise and honest judgment about what’s going on,” Raben said.

Last month, Monaco moved to the White House to become Obama’s chief counterterrorism adviser, replacing John O. Brennan when he became CIA director. When Monaco left, Holder said she had “no rival when it comes to her dedication to this nation, her experience in national security issues and her excellent judgment.”

Mueller will step down as FBI director on Sept. 4 after 12 years on the job.

If Obama selects Monaco to replace him, she will likely receive bipartisan support, a rarity in Washington. After she was named the president’s counterterrorism adviser, Michael B. Mukasey, a former attorney general in the George W. Bush administration, praised Monaco for her judgment, thoughtful decisions and deliberate style.

This week, that deliberate style was tested on the most high-pressure stage. On Tuesday, the day after the Boston bombings, the White House released a statement saying that the president was briefed by his top national security officials on all the developments in what was called a terrorist act.

The briefing, the White House said, was led by Monaco.

Scott Wilson, Julie Tate and Jason Horowitz contributed to this report.