A family of six trying to escape the floodwaters produced by Harvey has been counted among the 14 victims who died in the historic storm, authorities said.

The four children and their great-grandparents died Sunday afternoon when Samuel Saldivar, the driver of the van, attempted to pick them up amid the flooding, his sister-in-law Virginia Saldivar told the Associated Press. At one point, a strong current swept the van away and into the Greens Bayou.

"Sam calls my husband and tells him, 'they're gone,'" Saldivar said. "That's when my husband dropped the phone and started screaming."

The driver – the children’s great-uncle – managed to escape. He told everyone to get out of the van using the back door, but the van quickly sank, KHOU reported.

The victims include Manuel Saldivar, 84, and his wife Belia, 81; Daisy Saldivar, 6; Xavier Saldivar, 8; Dominic Saldivar, 14; Devy Saldivar, 16.

Witnesses said first responders were in the area, but they were too late to conduct a rescue operation, FOX26 reported.

"I'm just hoping we find the bodies," Saldivar said.

Their bodies were not recovered as of Monday night, but Harris County sheriff’s officials said "the 14 reported dead include a family of six who were lost when their van sunk after crossing a bridge and hitting high water." Saldivar said the Coast Guard told her family they couldn't search for the bodies until the water recedes. She added she has not yet told the children's father, her son, who she says is in prison for violating parole.

“Pasadena ISD's heart is heavy as we have learned that four Pasadena ISD students and two of their great-grand parents were swept away by the floodwaters of Greens Bayou while trying to escape the floodwaters of Harvey," the Pasadena Independent School District's statement read.

Texas officials said Monday that so far at least 14 people have died, but feared that the death toll likely to increase once the water level recedes and further search will be conducted, My Statesman reported.

Houston city officials said around 6,200 people are currently staying in shelters – with Houston Mayor Sylvester Turner claiming the city is after another facility to act as a “major shelter.”

“The need is tremendous,” Turner said, according to My Stateman. “No one would be here if they didn’t have to be here,” Turner said.

Texas Gov. Greg Abbott, in the meantime, mobilized Texas National Guard to respond to Harvey – calling up all 12,000 members to aid in search and rescue missions and other disaster relief efforts.

In a stunning show of Texas grit, volunteers are helping rescue Houstonians stuck in Harvey's path. Thirty to 40 people at a time are being loaded into dump trucks, trailers, boats and other large vehicles.

Flood victims are using their phone lights to signal volunteer rescue workers. Evacuees say 100 or more people are standing in the road, soaked to the bone, waiting for help.

Tens of thousands of people are still stuck as night falls and conditions on the ground worsen.

Harvey, which since has been downgraded to a tropical storm and is the most powerful hurricane to strike Texas in 50 years, reportedly moved into the Gulf of Mexico on Monday. Houston remains paralyzed and floodwaters reached many single-story houses in the city.

Further floods and torrential rains are expected, Reuters reports.

Louisiana, meanwhile, is bracing for its own flooding as the storm is likely to strike the state. Gov. John Bel Edwards alerted the people, saying “the worst is likely to come for us here” during a press conference Monday.

He added that Harvey “does remain a tropical storm and it’s going to drop an awful lot of rain” on the state of Louisiana, noting that “We do have a long way to go with this particular storm.”

Southwestern areas of Louisiana could see up to 20 inches of rain from the storm. The authorities have issued flood warnings last Thursday.

Fox News' Lukas Mikelionis, Barnini Chakraborty and The Associated Press contributed to this report.