“They got out and tried to get the ladies out,” Sheriff Thompson said. He was not sure how long they tried — possibly 45 minutes. They may have struggled because of the way the van was positioned against the guardrail or because of the pressure of the water on the doors.

The women were not being evacuated from floodwaters. They were both being taken from hospitals, where they had come voluntarily, to mental health facilities, where they had been committed. At the news conference, Sheriff Thompson said that his department had been responding to a court order to transport the women. On Wednesday, family members said they had heard nothing about any orders.

It is routine, and required under state law, for law enforcement to transport people who are involuntarily committed and who are determined by a physician “as posing an imminent risk of harm to him or herself by virtue of mental illness,” according to a statement from the state Department of Mental Health. It was unclear to the families whether the women had expressed an intent to harm themselves or anyone else.

The sheriff did not think the women were in restraints when they drowned, as early reports indicated. Restraining patients was not typical, he said, “if they’re not combative or having issues and I understand they were not.” But he could not say for sure.

The deputies also apparently drove onto a road that was blocked off because of flooding. The sheriff could not account for why they would have done that.

On Tuesday morning, Wendy Newton, 45, told her mother she could tell she was about to have what she called “a spell,” according to Ms. Newton’s daughter Allison. Ms. Newton asked to be taken to McLeod Hospital in Loris, S.C. An ambulance soon arrived to pick her up.