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Six members of the same family are believed to be dead after their van was swept away in floodwaters caused by Tropical Storm Harvey over the weekend, Houston-area television station KHOU reported Monday.

Two adults and their four great-grandchildren were trying to escape Sunday afternoon when their van was overcome, three relatives told the station.

The children’s great-uncle, who was driving the vehicle, managed to crawl out before the vehicle became fully submerged in water, KHOU reported.

Witnesses told the station the van had crossed a bridge in northeast Houston when it was hit with major flooding, which caused the van to be swept away.

The driver made it out and instructed the children to escape through the vehicle’s back door, but they were trapped and unable to as the van sank.

It is believed the family members drowned, according to KHOU.

The six people in the van were identified only as an 81-year-old woman and her 84-year-old husband; two girls, ages 16 and 6; and two boys, ages 14 and 8. None of the bodies have been recovered.

First responders were in the area at the time, but they did not have enough time to rescue the family, according to witnesses.

Authorities have not yet confirmed the incident.

As of Monday afternoon, there have been “six suspected flood-related deaths” in Harris County — which is where Houston is located — since Harvey made landfall last week, according to a spokeswoman for the Harris County Institute of Forensic Sciences.

Their manners of death will be confirmed later this week.

CNN contributed to this story.