Ricardo Portillo, the 46-year-old referee, is only the second official in the United States known to have died as a result of referee assault, according to the National Association of Sports Officials. But Barry Mano, the organization’s president, said that many serious assaults went unreported, and Portillo’s eldest , Johana, said her father had been assaulted before, sustaining broken ribs in another on-field attack about five years ago.