SEATTLE - A 37-year-old woman was killed when a power boat slammed into a sailboat on Lake Washington Saturday night.

Police say speed was a factor in the crash, but they don't yet know how fast the power boat, driven by a 17-year-old boy, was going.

Video: Woman killed in Lk. Washington boat collision

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Martyn Opacich saw the sailboat set sail Saturday night before dusk, its familiar owner at the helm.

He saw the captain sail back to Lakewood Marina before floating off again after dark.  Soon after, he heard the crash.

"I heard a big thud, boom.  I didn't think nothing about it, then I see some emergency vehicles here," he said.

Witnesses reportedly told police the passengers on the sailboat tried to get the power boater's attention.

"By waving their arms and yelling "stop, stop, slow down," said Renee Witt of the Seattle Police Dept.

Instead, police say, the powerboat smashed into the sailboat and actually rode up and over it.

Boater Frank New recalls what happened when officers arrived.

"The police jetted across the water on the other side of Seward and they went out of sight, and when they came back they were pulling a motorboat behind it," he said.

The powerboat is now locked up as police evidence. The boy who was driving it was released pending the outcome of the investigation.  His 16-year-old passenger had minor injuries.

Two other people on the sailboat were not seriously hurt.

Police are also investigating to determine if alcohol was a factor in the crash.