On Hokkaido Island Saturday evening, a woman and her three children, identified as Kazuyo Miyashita, 40, of Nakashibetsu and her children Misa, 17, Sayo, 14, and Daiki, 11, were heading home when they became stuck in the snow, The Yomiuri Shimbun reported.
Miyashita called an acquaintance at about 5:40 p.m. and asked for help. The person she called then alerted another person who went out and found the car at about 6:30 p.m.
Miyashita and her children were slumped over in their seats when he arrived.
Police said the car was mostly buried in snow when they arrived. The family most likely died of carbon monoxide poisoning since the car's exhaust pipe was found clogged with snow.
Police said at least 75 people in Hokkaido became stuck in the snow in their vehicles Saturday.
Investigators in Akita prefecture Sunday were looking into a train derailment caused by snow that left 130 passengers and crew stranded for six hours, the Yomiuri Shimbun said.
The train derailed about 4:05 p.m. Saturday between Jinguji and Kariwano along the Akita Shinkansen line.
The conductor of the train made an emergency stop after hearing a thump beneath the train, East Japan Railway Co.'s Akita Branch Office said.
Once the train stopped, the conductor got out and found that the front wheels on the right side of the first train car had gone off rails.
At the time of the derailment, the area was hit by a blizzard, which led to the train being stuck in place for about six hours.
None of the passengers or crew members was injured in the incident. Buses finally arrived at about 10:40 p.m. and transported passengers to their final destinations.
Authorities were investigating how the wheels came off the track. They determined this was likely due to snow build up.