When Boston met with Horford for two hours on Friday, the four-time All-Star was well-prepared and had peppered the team’s brass with questions. The Celtics, meanwhile, delivered a lengthy presentation.

The Celtics were pleased with the gathering, and as they boarded a private jet to fly back to Boston they were also thinking about forward Al Horford, their other prime target.

On Saturday evening, a deep Celtics contingent that even included Patriots quarterback Tom Brady wrapped up a recruiting meeting in New York with the prize of this free agent class, Kevin Durant.

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“It covered every aspect of life on and off the court in Boston and our team’s future,” said a team source. “We just made it clear from the beginning that there was a real connection.”


Still, the Celtics were unsure where they stood with Horford, particularly as the Atlanta Hawks — the team with which Horford had spent all nine years of his career — made a late push to re-sign him.

But as the jet prepared to take off, a phone rang and delivered the news the Celtics, their fans, and their city have been waiting for: Al Horford had accepted Boston’s four-year, $113 million maximum-salary contract offer. The pilot was asked to pull to the side of the runway, a source said, and when the call ended, there was a celebration.

After months of waiting and hoping and working, the Celtics had finally struck.

“We’re thinking big, and this is a big move,” the team source said. “This will set us up for the future in a better place.”

At 7:15 p.m., Horford posted on his Twitter account: “Celtic pride!!!!!!” He also changed his profile picture to the Celtics’ logo. He cannot officially sign with Boston until the league-wide moratorium ends on July 7.

According to a league source, Horford’s four-year deal has a player option after the third season.


Horford’s father, Tito, said in a telephone interview Saturday night that his son had narrowed his choices down to the Hawks and the Celtics. As the family discussed the options, the conversation often returned to the uncommon passion of Boston’s fans.

In Atlanta, support at home games was sometimes tepid, despite the fact that the Hawks had gone 108-56 over the last two years.

“There wasn’t as much motivation for him when he saw all the empty seats when they were winning,” Tito Horford said. “He said to me, ‘Dad, when we were playing Boston, they were down 15 points and they were cheering their team like they were winning the game. They’re so into the game.’ This is special for us, especially for him.”

Tito Horford, a center who played for the Bucks from 1988-90, said he was always impressed by the franchise’s rich tradition and unending support.

“Ever since Boston, Danny Ainge and the whole organization began to show interest in him, that opened his eyes,” Tito Horford said. “He always had a good feel about Boston.”

Horford’s decision could hold even more significance, too, as it comes on the heels of the Celtics’ meeting with Durant. With Horford and fellow All-Star Isaiah Thomas, Boston can now pitch its own version of a “Big Three.”

Even if Durant ultimately decides to sign a two-year deal with the Thunder in which the second year is a player option — long considered his most likely path — the Horford signing and Saturday’s meeting could send an important message for the future.


Tito Horford said his son did not mention the possibility of Durant joining him in Boston and stressed that it was not the reason he chose the Celtics.

“He just said ‘Right now, I’m excited,’ ” Tito Horford said. “He said, ‘I’m going to play for the Celtics, Dad. I don’t know who else they might get, but I’m here, and I’m hoping for the best.’ ”

Regardless of whether Boston ultimately signs Durant, the Celtics improved drastically on Saturday night.

The deal with the 6-foot-10 forward is perhaps the biggest free agent coup in the history of this franchise. Horford, 30, averaged 15.2 points and 7.3 rebounds this season. He is a strong defender who also made 34.4 percent of his 3-point attempts.

“I think that Al has signaled his intentions to be a Celtic is really a historic step for this franchise,” a team source said. “To take a 48-win team with a young core and a great coach, and you add a four-time All-Star as a max free agent, and you can just see the direction we’re going in.”

On Friday the Hawks agreed to a three-year contract with former Rockets center Dwight Howard. The move appeared to be a signal that Horford’s time in Atlanta may have reached its end.

But on Saturday morning multiple reports suggested that the Wizards had joined the Celtics as finalists in the Horford sweepstakes. Then later on Saturday night, there was believed to be growing momentum that the Hawks had reemerged as favorites to re-sign their star.


If Horford had spurned the Celtics’ overtures it would have been a tough blow, and the missed opportunity would have had a ripple effect that left the team scrambling, as most of the top free agents found new homes over the past few days. Instead, Boston found a new centerpiece who should help make it one of the elite teams in the Eastern Conference.

“I think it’s a great coaching staff and they have good players, and the tradition when you play for the Celtics, the Lakers, these are traditions,” Tito Horford said. “People are always going to support the Boston Celtics.”

Now, Boston’s entire focus turns toward Durant. The Celtics played a surprising card on Saturday by bringing Brady to their meeting with the 2014 NBA MVP. He was joined by Ainge, coach Brad Stevens, team co-owners Wyc Grousbeck and Steve Pagliuca, and Celtics players Thomas, Jae Crowder, Marcus Smart, and Kelly Olynyk.

A team source declined to go into detail about Brady’s role in the meeting, saying simply: “Tom did a very good job today.”

It may still be unlikely that Durant will choose Boston, but Horford’s presence can only help. If the Celtics can complete the impossible and add both Durant and Horford to their already talented core, they would immediately become NBA title contenders.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.