A tour boat carrying a group of senior citizens capsized and sank on an upstate New York lake Sunday, killing 21 passengers, local authorities said.

The Ethan Allen was carrying 48 passengers and one crew member when it overturned off Cramer's Point, on the western shore of Lake George, Warren County Sheriff Larry Cleveland told reporters.

The passengers were part of a Canadian-owned tour bus group of elderly people from Michigan, he said.

"Some were using walkers and wheelchairs and were not able to get around easily," he added.

The 40-foot boat was navigating calm, 70-foot-deep waters on a clear, 69-degree day when at 2:55 p.m. the vessel apparently rolled over after being hit by the wake of a larger craft, Cleveland said.

The boat is operated by Shoreline Cruises, based in Lake George, New York, about 50 miles north of Albany. Both the operators and the captain, who was not hurt, "are cooperating with us fully," Cleveland said.

"We do not believe there is anything criminal about what took place," the sheriff said.

Though all the survivors were taken to nearby Glens Falls Hospital to be checked out, only as many as six appeared to need immediate treatment, he said.

"I'm not aware of any such incident of this magnitude in 37 years I've been on this job," said Wayne Bennett, superintendent of the state police. "This is unprecedented."

Witness Andrea Sause said she assisted with the rescue efforts.

"I saw like 20 people that were laying on the grass that were covered. And they were dead," said Sause, who was cleaning rooms in a nearby building when she heard ambulances and went to investigate with her father.

"We were helping, bringing blankets, and we were pulling the elderly out of the boat," she told a reporter. "They were all frantic, and a lot of them had chest problems."

Her father, Frank Sause, said he helped take people from a rescue boat, and "most of them looked unconscious."

The survivors, he added, "were all pretty well shook up."

Joanne Rahal said she was on her boat with her children when they saw smoke and went to investigate.

"There was all these people in the water," she said. "A lot of them had lifejackets. Some of them didn't, so we were trying to throw life jackets and get as many people on our boat as we could."

Cleveland said it did not appear to him that anyone had time to put on a flotation device.

"I did not see anyone that had one on," he said.

Rescue workers attempted to turn over the boat in order to save anyone who trapped underneath, Rahal said.

"But then, when they did that, the boat started to sink all the way ... There was people floating in the water."

In all, she said, she took into her boat seven people, two of whom were unconscious. Her attempts at CPR did not appear to have been successful, she added.

Officials said the boat was about 40 feet long, with a single deck, a covered top and open sides.

A joint investigation by the sheriff, the state police and the Lake George Park Division will be carried out, Cleveland said.