New Hartford, New York (CNN)Christopher Leonard spoke quietly, his head bowed, as he described the day his brother died.

One minute, they were standing in front of their church and facing questions from the pastor. Then, he said, the beating began.

"It was a black cord about 4 feet long," he testified in court on Wednesday. "It was folded twice."

The 17-year-old could be a key witness in a case that started in a small church in upstate New York and has drawn national attention. Police say Christopher Leonard and his brother Lucas were beaten in the sanctuary of Word of Life Christian Church in New Hartford, New York, after Lucas expressed a desire to leave the congregation.

Lucas Leonard, 19, was later pronounced dead at a hospital. He suffered injuries so severe that emergency room doctors thought he had been shot, police said.

"He was laying on the ground moaning. ... I knew something wasn't right. ... I rushed over and saw he wasn't breathing, so I tried to give him CPR," Christopher Leonard testified Wednesday, describing the moments after the beating.

The accusations have shaken the tiny town about 250 miles north of New York City. The boys' parents, Bruce T. Leonard, 65, and Deborah R. Leonard, 59, are charged with first-degree manslaughter. Four other church members -- including Sara Ferguson, the brothers' stepsister -- were charged with second-degree assault, also a felony. All have pleaded not guilty.