Catherine Manson sits on the front porch with her children, Caydence Manson and Carter Manson, in Hartford Wednesday.

HARTFORD — Amid the balloons, cake and games at his best friend’s birthday party on a farm, 5-year-old Carter Manson clutched his small chest.

“He just kept saying ‘I can’t breathe, I can’t breathe,’’’ his mother, Catherine, recalled tearfully. “I picked him up and told him it was OK and to just breathe. Just breathe.”