A 17-year-old National Guardsman has confessed to the June 20 killing of ABC television newsman Bill Stewart, a Nicaraguan board of inquiry announced today.

Santiago C. Carrasco, an auxiliary who had served in the guard for three months, told investigators that he shot Stewart accidentally when his M16 rifle fired because the safety was not on, according to Lt. Col. Manuel Sandino, president of the National Guard board of inquiry.

"I was going to hit him in the head and the shot went off," Carrasco was quoted as telling investigators. "I didn't know he was a journalist."

Stewart was shot at a National Guard roadblock in eastern Managua. An ABC camera crew film of the killing showed that Stewart was forced to kneel and then shot in the head.

When first question by the Board, Carrasco denied knowing who killed Stewart, Sandino said. He admitted the shooting after being shown an ABC videotape of the incident, the colonel said.

Carrasco said he was told that Cpl. Lorenzo Brenes shot Stewart's interpreter, Francisco Espinoza, Sandino said. The colonel said Carrasco told investigators he initially lied because Brenes told him he would get others in the unit in trouble and he would go to prison.

Brenes had been questioned early in the investigation and had told the board that a Private Gonzalez had killed Stewart and that Gonzalez was killed in combat later in the day.