SAMMAMISH, Wash. -- Authorities are still trying to figure out what caused a 68-year-old woman to crash her car through her own home on the shore of Lake Sammamish Friday night. The woman's 70-year-old partner inside was killed, and her daughter and son-in-law critically injured.

After talking with King County sheriff's detectives overnight, detectives found probable cause of impairment and booked the woman into King County Jail early Saturday morning on charges of vehicular homicide and vehicular assault.

The crash happened just before 8 p.m. at a home on Eastlake Sammamish SE in the Waverly Shores neighborhood of Sammamish.

While officials were still piecing together the events, investigators say all people involved were related. The driver, a 68-year-old woman, was driving the family's Jeep SUV with her 3-year-old grandson.

For some reason, the jeep plowed though the family's home and into the lake, killing the woman's partner inside the house. The driver's 34-year-old daughter and 40-year-old son-in-law were on the dock and hit by the car; they were rushed to Harborview Medical Center with critical injuries.

The driver and the three-year-old child inside the car were not hurt.

My neighbor had her grandson in her lap just moving the car, put it into gear and the car shot forward through the house and ended up in the lake about 40 feet behind there, said neighbor Kevin Connelly.

A photo taken by a neighbor showed a white Jeep nose hanging over the edge of the dock, nose down into the water. According to neighbors, the Jeep was brand new and the woman may have had a problem with the gear shift.

From what Iunderstand from the neighbors, she was trying to operate this car and it wouldn't go into gear correctly and next thing they knew, it went right through the house, said a neighbor.

KING 5's Heather Graf and Alison Morrow were at the scene and tweeted updates and photos on Twitter at @HeatherGrafK5 and @AlisonMorrowTV.

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