An upstate New York tour boat was overloaded with passengers when it capsized on a fall foliage tour in October, killing 20 elderly tourists, federal investigators said Tuesday.

The extra weight made the 40-foot Ethan Allen dangerously unstable after it was struck by a wave from a passing boat or boats on Lake George in the Adirondacks, investigators from the National Transportation Safety Board said.

"It appears that there were a large number of passengers on board that seems to exceed the number that perhaps should have been," said Capt. Morgan Turrell, an NTSB investigator.

The boat was certified to carry 50 people — 48 passengers plus two crew — according to weight limits that have since come under scrutiny and been revised. There were 48 people on board when it capsized.

Turrell said investigators believe the boat, operated by Shoreline Cruises, was rocked by a wake or combination of wakes, but they could not determine the size of the wave. Investigators ruled out any leakage or other problems with the hull and operator fatigue.

The Oct. 2 accident — in which the boat suddenly flipped on a calm, sunny day — sparked changes in federal weight rules that could ultimately affect every commercial passenger boat in the United States.

The tragedy also had implications for the Coast Guard's decades-old rule calculating an average passenger weight at 140 pounds, which assumes a mix of men, women and children. New York state watercraft rules on weight followed the federal standard.

But Americans are about 25 pounds heavier than they were 40 years ago and in April the Coast Guard announced it would set a standard of 185 pounds per person. The new weight calculation is voluntary until new rules are created.