D.C. Police investigators were continuing their questioning yesterday of dozens of guests at a Saturday night wedding party to determine exactly what led to an exchange of gunfire that left a cousin of the bride dead and an off-duty police homicide detective critically wounded.

The dead man was identified as Samuel Elbert McFadden, 30, of 536 Edgewood St. NE. He was shot three times and died at the Washington Hospital Center about an hour after the 11:30 p.m. incident.

Homicide detective Ronald E. Washington, 40, a friend of the newly-wed couple, was in critical condition at Washington Hospital Center with gunshot wounds in the lower right abdomen and the upper right thigh.

Washington, a member of the police department for 12 years, has been placed on routine administrative leave pending a decision by the grand jury about whether he acted properly in exchanging gunfire with McFadden.

The wedding reception in the social room of an 11-story apartment building at 611 Edgewood St. NE had been going on for about 3 1/2 hours before the 11:30 p.m. shootings. According to guests at the wedding, the incident started when McFadden began arguing with another cousin at the reception room bar because McFadden didn't like her "attitude" towards him. When McFadden began swearing loudly at his cousin, witnesses said, the groom and a security guard asked McFadden to leave.

McFadden then pulled out a hand-gun and struck the groom on the head, sending him sprawling to the floor. The bride, who was standing nearby, began screaming.

McFadden ran from the room into the hallway, where he pointed his gun at a second security guard, according to police Insp. Melvin Winkelman.

Washington, informed of the melee by a guest, rushed into the hallway with his service revolver drawn, and, after identifying himself as a policeman, ordered McFadden to drop his gun, Winkelman said. McFadden turned from the guard and fired several shots at Washington, wounding him twice. Washington returned the fire, police said.

The groom, Willie A. Smith, 36, of 1204 Penn St. NE, and offset printer, was treated at Washington Hospital Center for a wound to his forehead and released.

Smith and his bride, the former Maggie Harrison, a secretary, postponed their two-day honeymoon to King's Dominion amusement park near Richmond as a result of the incident, according to wedding guests.

The guest, who had been dancing to disco records, chatting and eating cold cuts, were thrown into pandemonium by the incident. Chairs were knocked to the floor as some guests scrambled for cover and others ran to see what had happened. Other guests ran to pay telephones to call police.

"This isn't the kind of thing you expect to see at a wedding," said one of the 50 guests, Vivian Click, 31, of 815 Portland St. SE. "I couldn't believe what was coming down, with the bride screaming and everthing."

"My husband was holding Ron (washington) in his arms, patting his face. There was blood all over," she said.

Another guest, Barbara Blair, said the incident left her "all shook up." "I feel sorry for the bride," she said.

Homicide detectives said they had not determined whether Washington was in the party room prior to rushing into the hallway, but a guest said he had been sitting on a folding chair in the middle of the room, talking with other guests.

Washington is married and has three children. His family was not with him at the wedding reception. CAPTION: Picture, RONALD E. WASHINGTON . . . in critical condition