The bodies of the three Spaniards - two men and a woman - lay in pools of blood in the aid workers' house in Ruhengeri yesterday.
A United Nations human rights operations director, Javier Zuniga, said: "These people were executed. Clearly these attacks were aimed at aid workers and expatriates generally."
The American, named by aid workers as Nitin Madhav, was working with the Spaniards. He was treated in the hospital at Ruhengeri, where his wounded leg was amputated, and later was flown to Kigali and evacuated for further treatment.
An American diplomat said the attackers stormed into the house and demanded the Spaniards' passports. They were then disturbed by the noise of gunfire outside the house, and reacted by shooting the three from close range.
Mr Madhav, 28, from Pennsylvania, was shot in the leg as he dived behind a table for protection. The Spaniards, shot through the head, were attached to the Spanish charity Medicos del Mundo. In Paris the parent group Medecins du Monde announced that it was suspending all activities in Rwanda pending further information about the killings.
In Madrid, Medicos del Mundo named the dead as Manuel Madrazo, 42, a doctor from Seville, Maria Flors Sirera, 33, a nurse from Lerida, and a photographer, Luis Valtuena, 30, from Madrid who was working as an administrator.
Most expatriate staff in the Ruhengeri area left for the capital Kigali where the UN, relief groups and the government will meet today to decide whether to formally suspend humanitarian work on security grounds.
A Rwandan military officer said: "We know these people are now operating from inside Rwanda. They think if they can drive out the expatriates now working here they can mount such attacks more easily."