WASHINGTON — In the days before he pleaded guilty to lying to the FBI, and as he braced for the public revelation that he’d morphed from government target to government cooperator, Michael Flynn was reveling in the pleasures of a new grandchild, swapping cheerful observations about babies with a longtime friend and fellow grandfather.

At the same time, President Donald Trump’s former national security adviser also was lamenting the emotional toll of the criminal investigation he was facing, another friend recalls.

Flynn made clear “he was tired of the constant barrage of either people trying to contact him and his family, or the press and the coverage they were getting. He was ready for that to be over,” said Thomas A. Heaney Jr., a retired Army colonel who has known Flynn since childhood and spoke to him two days before the guilty plea last month.

People close to Flynn described to The Associated Press the pressures of the past year as investigators with special counsel Robert Mueller not only zeroed in on him but also intensely examined the dealings of one of his sons, Michael G. Flynn. The conversations offer a window into his thinking in the period before he entered his plea, which requires him to cooperate with Mueller’s probe into Russian meddling in the 2016 presidential election and possible coordination with Trump’s campaign.

A chance to end the case through a single-count guilty plea was almost certainly the most favorable outcome possible, especially since the aggressive, wide-ranging nature of Mueller’s inquiry made criminal prosecution a near-inevitability and a tough prison sentence a vivid possibility. Even before Mueller’s appointment, the FBI had been investigating Flynn’s conduct, including his conversations with the Russian ambassador and payments from a Turkish businessman for lobbying work.

In an indication of the value of his cooperation, prosecutors charged him only with false statements and set a guideline range of zero to 6 months in prison. His son, who had a baby last spring and worked alongside his father on the Turkish lobbying and other matters during the campaign, was notably not charged.

Flynn’s lawyer, Robert Kelner, declined to comment for this story.

The pressure on Flynn, 58, and his family was evident as prosecutors made him central to their investigation into Russia and the Trump campaign. He was the first White House official charged in the inquiry.

Michael Ledeen, another close friend who co-authored with Flynn, “The Field of Fight: How We Can Win the Global War Against Radical Islam and Its Allies,” said Flynn seemed delighted after Thanksgiving about the birth of Flynn Jr.’s baby. But Ledeen also detected stress, saying Flynn told him his wife’s mother was ill and how he was planning to sell his home in the Washington area to move to Rhode Island.

“He said it was a terrible time for him,” Ledeen said.

Though it’s not clear what information Flynn is prepared to provide, his cooperation with Mueller could cause a fissure in a relationship with Trump that had been strong.