The report came in shortly before 1 p.m.: A child playing on a mounted cannon at Castle Island had fallen, cutting his head. Boston police Officer Patrick J. Carr and his colleagues were tasked with accompanying an ambulance across the bridge. It was 100 years ago this week.

As the vehicle traveled over what was then a wooden drawbridge, one of its wheels crashed through a plank. The ambulance lurched into a fence, tossing Carr through the windshield and into Pleasure Bay. He died at a hospital on Aug. 1, 1916, despite colleagues’ efforts to save him.

A century after his death, Boston police officials gathered with Carr’s descendants — three of whom have followed him into the force — for a wreath-laying to honor his sacrifice.

“It’s unbelievably wonderful,’’ said Mary Coyle, Carr’s granddaughter. “All of us are touched beyond words.’’

A modest memorial for Carr had been affixed several years ago to a utility pole near the spot where he died. The wooden bridge has long since been replaced by a roadway of steel and concrete, but a placard bearing his name still looks out on the expanse of water.

Police Commissioner William B. Evans noted at the ceremony that the monument is not far from the spot where his brother, Joe, was killed when Evans was a boy. It’s a place for reflection, said the commissioner, who lives nearby and often walks in the area.

“Every time you walk by here, it brings back the memory of somebody who really made the ultimate sacrifice,’’ Evans said.

Family members said Patrick Carr and his wife, Mary Ellen Burke Carr, were both recent immigrants from Galway, Ireland. He left seven children when he died, the oldest of whom was only 10 years old.

The City Council awarded the family $300 per year, according to a Globe article at the time, but that, at less than $6 per week, was “entirely insufficient to care for the simplest needs of the family.’’

Complicating matters was the 1919 Boston Police Strike, which Coyle said disrupted Carr’s pension. For a time, she said, it was as though he never existed. Family members always kept his service in mind, however, and when the memorial went up, Coyle said, “we couldn’t believe it.’’

Patrolman Christopher Carr, who has been on the force for seven years, is the great-grandson of Patrick Carr. He said he has passed by the monument a handful of times.

He said Patrick Carr’s story inspired him, but so did the service of other family members. His father, James Carr, is a retired detective. Another grandson of Patrick Carr, James Keegan, is a retired harbor patrol officer.

As he looked around the event Monday, Christopher Carr said he did not expect such a large event. More than a dozen family members were there and so were many law enforcement colleagues. “With the family’s history, that’s pretty much what drove me to be a police officer,’’ he said.

Andy Rosen can be reached at andrew.rosen@globe.com.