"They were way too close to the dam all day," said Albert Pulice Jr. of Murrysville, who was fishing from the riverbank in Harmar.
Several people who gathered at the Pennsylvania Fish and Boat Commission marina about a mile downstream said buoys usually indicate an area just below the dam as too risky for boats.
The buoys are removed during the winter and have not been re-installed.
Pulice's friend, Pat Thomas of Murrysville, said it appeared a fishing line snagged and pulled the boat even closer to the dam.
"The nose of the boat hit the wall and they were caught in the wash," said Thomas, referring to the water flowing over the dam and churning at its base, which can trap objects against the wall.
Thomas said the water pouring off the dam capsized the boat almost immediately. He said none of the boaters were wearing life jackets.
"Everybody on shore was yelling for (the survivor) to keep swimming," Thomas said. The man eventually made it shore well downstream of the dam.
Thomas said the other two victims initially clung to the boat but soon were forced underwater.
The Allegheny County Medical Examiner's Office did not have information as to the cause of the men's deaths late last night. In addition to the threat of injury and drowning, the boaters were at risk of hypothermia after prolonged exposure to the cold water temperature. The water was about 50 degrees.
The men's bodies were found about 100 feet downstream nearly a half-hour after the accident, police said.
Thomas said volunteers arrived at the riverbank almost immediately and rescue boats were in the water minutes later.
The victims' overturned boat remained trapped in the churning water at the base of the dam but had drifted toward the lock on the Plum side of the river.
Thomas said he had considered yelling to the boaters to move away from the dam, but didn't think they would be able to hear him over the roar of the water.
Waterways Conservation Officer Larry Furlong said the Fish and Boat Commission is investigating.