(CNN) -- The four people killed in a helicopter crash on a highway in Maryland on Thursday night were returning from a youth event and had delayed takeoff because of weather, a National Transportation Safety Board member said.

Kitty Higgins said at a news conference that the four were returning to Frederick, Maryland, from an event for the Advanced Helicopter Youth Foundation in Hagerstown, Maryland.

The foundation says on its Web site that it aims to "heal families with an at-risk child before a crisis is reached in the home."

The helicopter crashed about 10:30 p.m. Thursday on Interstate 70, prompting authorities to close the highway for several hours.

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Higgins said the pilot, who had about 630 hours of flight experience, waited about two hours to take off from Hagerstown because of weather conditions.

She did not provide further details on the weather. The helicopter took off about 10:15 p.m., she said.

There were three passengers on the flight, Higgins said: two employees of Advanced Helicopter Concepts and a friend of theirs.

Witnesses said the helicopter was flying low over the highway before it hit power lines, and then they saw sparks, Higgins said. She said the craft burst into flames after it crashed.

While in Hagerstown, the pilot had given several flights to raise money for the Advanced Helicopter Youth Foundation, Higgins said.

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