ST. LOUIS -- Video and audio recordings from a fatal duck boat accident in Missouri show that the lake went from calm to deadly dangerous in a matter of minutes, the National Transportation Safety Board (NTSB) said Friday. The agency cited preliminary findings gathered from the video recorder camera system salvaged by divers after the duck boat sank July 19 at Table Rock Lake near Branson.

Seventeen of the 31 people on board died, including nine members of a family from Indianapolis.

The material was examined at a lab in Washington, but the agency has not yet analyzed the findings and no conclusions about the cause of the accident, one of the nation's worst maritime accidents in recent decades, can be drawn. The findings, though, paint a chilling picture of the final few minutes before the boat went under.

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The captain and driver boarded the vessel at 6:27 p.m. The excursion begins on land at a terminal in Branson. Normally, the vessel tours the popular country music and entertainment community first before going to the lake for about a 20-minute boat ride. The driver drives the vehicle on land, and the captain takes over on the water.

But the video recordings show that at 6:28 p.m., someone briefly stepped onto the rear of the vehicle and told the crew to take the water portion of the tour first. A minute later, with passengers boarding, the captain made a reference to looking at the weather radar prior to the trip.

The vessel arrived at the lake a few minutes before 7 p.m. and the captain briefed passengers on the location of emergency exits and life jackets, then demonstrated use of life jackets and pointed out the location of life rings. The vessel entered the water around 6:55 p.m. at a time when the water appeared calm, the NTSB said. In fact, over the next five minutes the captain allowed four different children to sit in the driver's seat.

But suddenly just after 7 p.m., whitecaps rapidly appeared on the water and winds increased, the NTSB said. The captain returned to the driver's seat. The driver lowered plastic side curtains and at 7:01 p.m. the captain made a comment about the storm.

At 7:03 p.m. the captain made a call on a handheld radio but the content was unintelligible. A minute late, an electronic tone associated with the bilge alarm activated, until about a minute later when the captain reached down and the alarm stopped.

The captain made another call on a handheld radio at 7:05 but the content was again unintelligible. Over the next couple of minutes, water splashed inside the passenger compartment.

At 7:07 p.m. an electronic tone associated with the bilge alarm activated again. At 7:08 p.m. the inward-facing video recording ended as the vessel was still on the surface of the water.

Phone and email messages left with a spokeswoman for Ripley Entertainment, the owner of Ride the Ducks of Branson, were not immediately returned.

The company's president, Jim Pattison Jr., told "CBS This Morning" last week that the boat shouldn't have been in the water. "This business has been operating for 47 years and we've never had an incident like this or anything close to it, he said.

A private inspector who examined 24 duck boats for Ripley Entertainment in August, including the one that sank, said that when the bilge alarm went off, it would be a sign that, "There's a significant amount of water in the hull."

"It just wasn't getting evacuated," said Steve Paul, owner of Test Drive Technologies in the St. Louis area.