NEW BRIGHTON, Minn. (AP) _ A seaplane taking off from a lake crashed Sunday when a wingtip struck the water, killing all five people aboard, authorities said.

Killed in the crash on Long Lake were the pilot, a man and his two children and the man's girlfriend, said Ramsey County Deputy Sheriff Darwin McVary.

The crash occurred about 9:50 a.m. Sunday when the pilot, Herman Behrens, 73, turned the plane shortly after takeoff and one of the plane's wings caught the water, McVary said.

The plane crashed into the water nose first, and flipped upside down, leaving only its pontoons above water, said Wanda Wolf, a lakeside resident.

''For a reason yet undetermined the aircraft pitched nose up and then nose down and then went into the water,'' said William Bruce, an air safety investigator with the National Transportation Safety Board in Chicago.

''Now we have to find out why.''

The Federal Aviation Administration also is investigating, but FAA officials said it would be several days before they knew what caused the crash.

Other victims were identified by McVary as Coon Rapids residents Francis Leahan, 31; his two children, Kelly, 3, and Ryan, 6; and Leahan's girlfriend, Amy Westlund, 27.

Behrens lived on Long Lake, where he kept the seaplane moored and frequently took guests on Sunday pleasure flights, Mrs. Wolf said.

Several lakeside residents who saw the accident tried to rescue the victims. It took authorities about an hour to free the victims.