Peter Guza jolted awake when the phone rang. It was the start of his junior year at Lehigh University. His first class wasn’t until 10 a.m., and the 20-year-old engineering major was sleeping in. As he looked around his tiny frat house bedroom, the shade drawn against the morning sun, he wondered: Who would be calling before 9 a.m.?

It was an old friend in New York, urging him to turn on the television. The caller knew what all of Guza’s old friends knew: His father, Phil, a 54-year-old mathematician for Aon Corp. , worked in the South Tower of the World Trade Center.

Downstairs, in a room he rarely visited, a half-dozen of his Beta Theta Pi fraternity brothers had gathered around the TV set. Sitting there beside them at 9:03 a.m., with empty beer cans on the coffee table, Peter Guza watched the second plane hit the South Tower. His heart dropped. But then, as he studied the point of the impact, he felt certain it was well below the 105th floor where his father worked.

People in the hallway murmured in low voices, spreading the news about his personal connection. More students pressed into the room. Guza reassured his friends; his father was probably fine. The minutes ticked by; it was almost 10. Then, inside a column of black smoke, the building lurched. Guza watched in silence as his father’s tower fell.

It was the end of one story, and the start of another. Thousands of families suffered devastating losses on Sept. 11, 2001. In Boston, the names of 209 victims with ties to Massachusetts are inscribed on a memorial in the Public Garden. Of those, 92 lived here when they died. They were the parents of more than 40 children.

Children lost parents and parents lost children. Brothers lost sisters and husbands lost wives—and every one of them had to find a way forward. Some grew more political; others withdrew. Some remarried; others divorced. They have moved, started businesses, changed careers, given birth. Still, their grief endures. Guza, now living in North Andover, feels the stab of loss when he plays with his 1-year-old son. In New Hampshire, Andrea LeBlanc feels it when she learns something new and exciting. Instinctively, a decade after her husband was killed, her first thought is still, “I can’t wait to tell Bob.’’

“People think either you’re grieving or you’re OK, and that isn’t true,’’ said Deborah Rivlin, a bereavement educator who worked with local 9/11 families for years. “The loss is forever, you are changed forever, but you learn to integrate the loss into your life. There’s an opportunity that comes, in finding ways to make meaning of the loss.’’

After the tower collapsed, sending roiling plumes of ash and dust aloft, Guza sat stunned in his college fraternity house. A Beta Theta Pi brother he wasn’t especially close with pushed his way across the crowded room and hugged him. A friend drove him home to Fair Haven, N.J., two hours away, the next day. That night, still hopeful that his father could be alive in a hospital somewhere, Peter, his mother, and brother made a flier with Phil’s picture on it. The clerk at the print shop didn’t charge them to make copies.

Later, Peter and his family would speak with colleagues who had talked to his father at work that morning. Aon Corp., a Chicago-based insurance brokerage, had 1,100 employees in the South Tower: 176 were killed. The company also had an office in Boston, where Peter’s brother, Tom, worked.

Phil Guza had called Tom that morning and left him a voicemail after the first plane hit the North Tower. There was no drama, in his tone or in his message. “There’s a big accident in Tower 1,’’ he said. “Everything’s fine here—back to work.’’

Phil Guza was a brilliant man, a MENSA member with a PhD from Princeton. He was also eccentric and stubborn; he paid no mind to trends and hated wasting time. To his family, the fact that he had stayed in his office was more than believable. It felt inevitable.

“He was a classic workaholic,’’ Peter Guza says. “Knowing his work ethic, there’s not a shred of doubt. . . . He was still doing his work. He probably worked to the end.’’

It was not the first time the family had sustained a sudden, inexplicable loss.

Nine years earlier, when Peter was 11, his brother Dave had committed suicide.