When Michael K. Atkinson addressed senators at his confirmation hearing last year to become the watchdog for the nation’s intelligence community, he promised a robust program to encourage whistleblowers — one that “validates moral courage without compromising national security and without retaliation.”

Atkinson returned to Congress on Friday after the alarms he raised from a whistleblower set in motion the House impeachment inquiry into President Trump’s phone call with his Ukrainian counterpart. Atkinson is also battling for legal protections for the unnamed person who issued the complaint, whom the president has called “fake” and “vicious.”

As a former federal prosecutor in Washington, Atkinson is well suited to his high-pressure role at the epicenter of the unfolding investigation, according to former colleagues. He helped send two Democratic congressmen to prison and oversaw a sprawling campaign finance investigation involving a former D.C. mayor. Atkinson is unflappable, understated and cautious, they say, but also unafraid to speak up when he disagrees.

“He plans many steps ahead; he’s not impulsive. He had to have known that taking this action was going to have repercussions,” said former colleague Matthew Jones, who was chief of staff to Atkinson’s boss, then-U.S. Attorney Ronald C. Machen Jr. “He’s not a person who goes out on a limb in any way.”

Trump picked Atkinson for inspector general of the intelligence community in late 2017, and he was confirmed in May 2018.

Sixteen months later, Atkinson was locked in a stalemate with the Justice Department and the acting director of national intelligence, who had refused to transmit the whistleblower’s complaint to Congress within seven days as required by law.

The law requires Atkinson to notify Congress when he is “unable to resolve” a difference with the intelligence director on a matter involving one of the inspector general’s duties. He sent two letters to Congress in September, informing the intelligence panels that a complaint existed, and told the committees he was continuing efforts to obtain direction from acting director Joseph Maguire as to how the whistleblower could bring concerns to Congress.

Atkinson then testified in a closed-door hearing, and was back for a follow-up closed-door session Friday with the House Intelligence Committee.

“It could have all been squashed by the Justice Department, but he persisted,” said Mary McCord, who worked closely with Atkinson when she led the Justice Department’s national security division and Atkinson was her senior counsel, and when both were in the U.S. attorney’s office in Washington.

“As soon as I saw that this was coming from him, that was all I needed to know to credit the report, because I know him to be a careful, thoughtful, meticulous prosecutor,” McCord said. “He doesn’t seek the limelight. He doesn’t seek attention.”

Atkinson’s notification prompted a subpoena from House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) to Maguire to turn over the complaint. Several days later, media reports revealed the complaint centered on Trump’s phone call with the Ukrainian president. On the call, Trump repeatedly pressed Volodymyr Zelensky to investigate former vice president Joe Biden and his son Hunter Biden.

The disclosures touched off calls from Congress for the White House to release a summary of Trump’s phone call — and for the whistleblower’s complaint to be made public.

Earlier this week, Atkinson pushed back against unsubstantiated allegations in conservative media outlets — and picked up in a tweet by the president — that the whistleblower submission form had been altered to enable complaints that were not based on firsthand knowledge. The whistleblower had acknowledged in the complaint not being “a direct witness” in most cases.

In a statement posted online, Atkinson said the form has been in place since May 2018. There is no requirement in the law, he noted, that a complainant possess firsthand knowledge.

Whistleblower attorney Bradley Moss said Atkinson, by “actions including providing official counterpoints to the political spin coming out of the White House and its media allies, has done more than anyone ever imagined that a politically appointed IG could have done.”

Atkinson, 55, grew up in New York’s Oswego County. When asked in his Senate confirmation questionnaire whether he had ever run afoul of the law, Atkinson, known as a straight arrow with a dry wit, wrote that as a high school senior, he paid a $75 fine for illegally fishing for trout “out of season, at night, and with a spear.”

Atkinson graduated from Syracuse University, and after law school at Cornell University he spent 11 years as a lawyer in private practice in Washington. He and his wife, the managing partner at a D.C. law firm, have two sons and live in Maryland, according to the Senate form.

It was after the 9/11 attacks that Atkinson decided to join the government.

“I wanted more challenges, greater responsibilities and different rewards,” he said at his January 2018 hearing. “Although my annual salary was reduced by nearly two-thirds from my time at the law firm, my sense of professional accomplishment was never higher.”

Inside the intelligence community leadership, Atkinson is viewed as “quiet, reserved” and one you “trust with investigations,” said one senior U.S. intelligence official who was not authorized to comment on the record and spoke on the condition of anonymity.

During his initial closed-door testimony before the House Intelligence Committee last month, Atkinson focused largely on what the law requires of his office. He said he interviewed witnesses to help determine the complaint’s credibility but did not identify the witnesses he interviewed, said a person familiar with the session.

Atkinson was “credible,” said the individual, speaking on the condition of anonymity to describe a closed session. He was “emotional in the sense that he was disturbed by being stymied and felt like he wasn’t able to do his job,” the person said.

Atkinson brought to the task his 15 years of investigating and taking on complex, high-stakes corruption cases involving elected officials, corporate executives and government contractors. He helped prosecute former congressman William Jefferson (D-La.) in a bribery case that uncovered $90,000 hidden in foil in Jefferson’s freezer.

His work led to convictions of the sons of two prominent Democrats. Jesse L. Jackson Jr., the former Illinois congressman and son of the Rev. Jesse L. Jackson, pleaded guilty in 2013 to stealing hundreds of thousands of dollars in campaign money to pay for an extravagant lifestyle. Former D.C. Council member Michael A. Brown, son of former commerce secretary Ronald H. Brown Jr., pleaded guilty the same year after he was photographed taking wads of cash in a gym bag and coffee mug from undercover federal agents.

On the other side of the courtroom from Atkinson in both cases was defense attorney Reid Weingarten. He said Atkinson, unlike other prosecutors who might have sought an overly harsh sentence or leaked details to the media, “handled himself with class, intelligence, with sensitivity and with strength.”

“The evidence was dreadful, but there was a sensitivity,” Weingarten said. “He didn’t rub it in my face.”

As part of a far-reaching campaign finance investigation, Atkinson negotiated the plea agreement with former D.C. contractor Jeffrey Thompson. He alleged in 2014 that then-Mayor Vincent C. Gray (D) knew about Thompson’s unreported, illegal “shadow campaign” on Gray’s behalf. Gray, now a D.C. Council member, was never charged, and he accused Thompson of lying. His supporters called the plea a sweetheart deal and criticized prosecutors for the timing — publicly naming Gray just weeks before he lost his primary bid for reelection.

Atkinson disclosed the identity of “Mayoral Candidate A” in court only after he was ordered to do so by the judge.

He was also the driving force in exposing a network of corrupt public officials and government contractors for the Army Corps of Engineers who stole more than $30 million through inflated billings and fake invoices.

At the sentencing for the mastermind, Atkinson urged the judge to send a message to thousands of public officials and contractors who must decide daily “whether to give in to the temptation of government corruption.”