Kenichi Fujitaki, 35, made sure he opened the window just enough to let water stream into the car his adopted son Hiroshi Umino used to drive off a cliff and into the sea last October, killing himself, police said.
Fujitaki is also accused of having spent considerable time checking out coastlines to find the place where Umino could kill himself easiest.
Police suspect Fujitaki brainwashed the suicidal Umino into taking his own life in the car with an insurance policy payout of 25 million yen going to Fujitaki if somebody died inside it.
Umino drove the car off a port in Hidaka, Wakayama Prefecture, on the night of Oct. 13 last year, dying at age 34.
Police said that in the days leading up to Umino’s suicide, Fujitaki had opened the driver’s side window of the car several centimeters. This was just enough space to ensure a steady stream of water to flow into the vehicle, but not enough to allow Umino to escape if he had any second thoughts about taking his own life.
Fujitaki, police said, also checked out several ports in the region in the days before Umino took his deadly plunge. Investigators said Fujitaki took into account such factors as wave-breaking blocks, the number of people around and the depth of the water before deciding on Hidaka as the place he recommended Umino to take his own life.